The US Army has used a designation system since the 1920s whose fundamentals have remained broadly unchanged since. This system consists of an item name which describes the type of vehicle followed by a sequential model number, or M-number. The item name consists of a basic name which establishes the concept of the vehicle, followed by one or more modifiers to better differentiate it. Thus the formal designation of the famous M4 Sherman tank is Tank, Medium, M4 while that of the M3 Stuart is Tank, Light, M3. Converted vehicles like tank destroyers and self-propelled guns were known as 'motor carriages' depending on their type of armament (gun, howitzer, mortar) and/or gun mount (multiple gun, combination gun), examples being the Carriage, Motor, 3-inch Gun, M10 tank destroyer, the Carriage, Motor, 105-mm Howitzer, M7 self-propelled howitzer, and Carriage, Motor, Multiple Gun, M16 self-propelled anti-air gun (with a quad mount). Modifiers were typically limited to information relevant to the type of vehicle. Thus, while motor carriages typically described their armament, cargo trucks would describe their weight class and drivetrain as was the case with the Ford GPW (Jeep) which was designated Truck, 1/4-ton, 4 x 4 (like many trucks, it had no M-number assigned). Amphibious vehicles are notable for following a slightly different convention, receiving an abbreviation of their item name and a mark number in place of an M-number. An example of this is the Landing Vehicle, Tracked, Mark I, LVT(1). Armored versions made reference to this in parenthesis, as was the case with the Landing Vehicle, Tracked (Armored), Army Type, Mark II, LVT(A)(2).
Informal convention for writing vehicle designations is usually in the form of the M-number first followed by the major modifiers, such as M4 Medium Tank, M10 Gun Motor Carriage, or M7 Howitzer Motor Carriage. Amphibious vehicles usually have a dash representing the mark number in order to avoid the somewhat unsightly use of two parenthesis, as is the case with the LVT-1 and LVT(A)-2.
During the development stage of a vehicle, a test number or (T-number) was used in place of the model number and followed its own sequence. So for example, the test number of the M4 Sherman was actually the T6. Some T-numbered vehicles entered service, these usually being conversions that were meant as stop-gap versions as was the case with the myriad of conversions of the M3 half-track such as the T19 and T30 self-propelled howitzers, some of which eventually were given an M-number (the T12 tank destroyer became the M3). It was decided around 1943-44 to homologize T- and M-numbers resulting in a notable jump in the M-number sequence. This is why the successors to the M4 and M5 tanks all had M-numbers in the twenties, these being the M24 and M26 tanks which now corresponded to their T-numbers. Vehicles built for foreign users like T17 armored car were not assigned M-numbers.
Variant designations were given an A-number suffix. This was applied only from the second main variant onward. Thus, the M4A1 is the second variant of the M4 tank, the original variant being designated simply M4. Armament changes based on the same chassis typically required the new gun caliber referenced in parenthesis following the A-number. The upgunned M4A1 tank was therefore known as the M4A1(76)W, the (76) being a reference to the new gun caliber, while the M4A1(105) was a version armed with a 105-mm howitzer for infantry support. Equipment or mechanical changes that were not deemed significant enough to warrant a new variant number were usually appended to the designation in an ad hoc manner, usually an abbreviation. In the above case, the 'W' in M4A1(76)W referred to the use of wet stowage in the tank's ammunition lockers. Meanwhile, those which switched their vertical volute spring suspension to horizontal volute spring suspension were known as the M4A1(76)W HVSS. These changes were also reflected in their full item name, this version of the M4 being the Tank, Medium, M4A1 (76-mm Gun, Wet).
In contrast, a change of chassis for a motor carriage resulted in the use of a B-number suffix. For example, the M7 self-propelled howitzer's initial variant was based on the M3 medium tank chassis but was later changed to that of the M4, resulting in the M7B1. Finally, an E-number suffix was used for experimiental modifications of existing variants, this being appended to the A- or B-number. It was not uncommon during wartime for these to be used in combat. For example, the M4A3E2 was known as the 'Jumbo' variant of the M4 tank fitted with extra armor and a new turret and it was used (with its designation unchanged) all the way up to the Korean war. The M4A3E8 would also be known as the 'Easy Eight' in reference to its E-number, and this nickname persisted even after it was formally redesignated M4A3(76)W for service. Note that a major change in the role of a vehicle would typically warrant issuing a new M-number.
Despite the fact that most major US vehicles of World War II are best known by the public by their colloquial names like 'Sherman tank', it might come as a surprise to know that these were never official. In fact, these names were not even given by US authorities but rather by the British which acquired most of them through Lend-Lease. A number of conventions were used for these names. Tanks were typically given the name of a a well-known US general (Stuart, Lee, Grant, Sherman, Chaffee), armored cars were given the name of hounds (Greyhound, Staghound), and self-propelled guns followed British convention of an ecclesiastical name (Priest). However, there were numerous vehicles that were never named, including some widely used ones like the M2/M3 half-track. The success of British-assigned names eventually drove the US Ordnance Department to eventually issue official colloquial names to major vehicles later in the war, with the use of US generals for tanks (starting with the M26 Pershing) remaining the de facto naming convention continuing up until today. Additionally, the nicknames of certain vehicles became so widespread that they became de facto names, as was the case with the Hellcat tank destroyer.
The main deficiency of the system was the sequential nature of M-numbers only within their item categories. This resulted in a glut of similar M-numbers referring to completely different vehicles, thus making the M-number useless on its own without an accompanying description. The M3, for example, could refer to a scout car, half-track car, light tank, medium tank, gun motor carriage, and tractor crane. Artillery pieces were particularly egregious offenders since the M-number was sequential only within the same types of guns with the same caliber. Thus, the M1 could refer to a 75-mm howitzer, a 155-mm howitzer, a 240-mm howitzer, as well as both an 8-inch howitzer and an 8-inch gun.
A list of the most common item names with some examples is provided below:
|Car, Armored, Light||M8|
|Car, Armored, Utility||M20|
|Car, Half-Track||M2, M3|
|Carriage, Motor, Gun||M3, M10, M12, M18, M36|
|Carriage, Motor, Howitzer||M7, M8|
|Carriage, Motor, Mortar||M21|
|Carriage, Motor, Multiple Gun||M15, M16, M17|
|Tank, Light||M3, M5, M22|
|Tank, Medium||M3, M4, M26|
|Landing Vehicle, Tracked||LVT-1|
|Landing Vehicle, Tracked (Armored)||LVT(A)-2|
|Landing Vehicle, Tracked (Unarmored)||LVT-2|
|Truck, Amphibious||GPA, DUKW|
The designation system changed in the post-war period sometime after the Korean War,* with the most notable difference being the elimination of the T-number, B-suffix and the expansion of the item name to include more information about the vehicle. The format for the item name remained mostly unchanged, with the only notable difference being the habit of using more modifiers. For example, tanks now had their propulson systems and gun calibers specified in the item name which was not usually the case for their World War II counterparts. Thus the formal designation of the M1 Abrams tank is Tank, Combat, Full-tracked, 105-mm Gun, M1. Note that the colloquial name of the vehicle, in this case General Abrams, is not included in the formal designation although it is sometimes appended in certain references after the M-number which would result in Tank, Combat, Full-tracked, 105-mm Gun, M1, General Abrams. However, this does not appear to be standard practice. The post-war system also did away with the 'motor carriage' concept, instead simply making reference to the self-propelled nature of a particular artillery system, an example being the Howitzer, Medium, Self-propelled, 155-mm, M109. For the most part, support and indirect-fire vehicles are not given colloquial names. This has been the case even for some of the most widely used post-war US vehicles like the M113 armored personnel carrier. In some cases, only certain variants of a vehicle were named as is the case with the M109A6 Paladin, the only variant of the M109 that has been named. As with World War II, informal convention when writing vehicle designations is simply the use of the M-number and either the colloquial name (if assigned) or major modifiers, for example, M1 Abrams or M109 Self-propelled Howitzer.
Another highly welcome change was that M-numbers became sequential outside of the main item categories, essentially becoming a single series. This process had already taken its first step when T- and M-numbers became homologized but now applied to all item categories within a broader category such as vehicles or guns. This was particularly useful for some of the most problematic categories under the previous system such as artillery. Unlike most other equipment, a considerable number of World War II-era artillery systems remained in use well into the Cold War and these were renumbered to make them consistent with the new sequence, for the example the 8-inch Howitzer M1 became the 203-mm Howitzer M115 (millimeters have since been used exclusively for gun calibers).
The post-war designation system retained the A-suffix for identifying variants as well as the E-suffix for identifying experimental modifications. The A-suffix now included all approved modifications, including those that were merely limited to the chassis and which were previously designated by the B-suffix. However, the retirement of the T-number required a new way of identifying vehicles in their development stage, this being the XM-number. As such, the prototype of the M1 tank was known as the XM1. In order to avoid the type of confusion that existed when T- and M-numbers did not match, the M-number would always be equal to its XM-number once a vehicle was approved for production. Examples of variants of the M1 tank include the M1E1 testbed for the Rheinmetall 120-mm gun, as well as the M1A1 and M1A2 which have been the second and third main production variants. In contrast to the relatively straightforward designation of variants, the designation of sub-variants is somewhat more arbitrary. This typically takes the form of an abbreviation of the new equipment or improvement program. For example, uparmored versions of the M1A1 are known as the M1A1HA (Heavy Armor) while a comprehensive improvement program of the M1A2 resulted in the M1A2 SEP (System Enhancement Package) with a further set of improvements resulting in the M1A2 SEPv2. A testment of the arbitrary nature of sub-variant designations is that the third and fourth improvement programs resulted in the M1A2C and M1A2D (although SEPv3 and SEPv4 are sometimes used as well).
Conversions of basic vehicles result in a change in the item name and, occasionally in the type designation (M-number) and colloquial name. For example, the Launcher, M60A1 Tank Chassis, Transporting for Bridge, Armored Vehicle Launched, Scissoring Type, Class 60 is the full designation for the bridgelayer variant of the M60 tank, which is more commonly known as the M60 AVLB (Armored Vehicle Launched Bridge). However, the combat engineer vehicle was given a new M-number and is known as the Vehicle, Combat, Engineer, Full-tracked, M728 or M728 CEV (Combat Engineer Vehicle). Some examples of variants with changed colloquial names include the M104 Wolverine bridgelayer variant of the M1 tank, and the M6 Linebacker self-propelled air defense variant of the M3 Bradley cavalry fighting vehicle. Colloquial name changes are more frequent when there are changes in weapons systems.
Despite the sequential nature of the M-numbers, one of the most notable trends in the post-war period was the "reset" that began with the M1 Abrams and continued with the M2 and M3 Bradley fighting vehicles. Although this would appear to apply to major new combat vehicles, a number of support and experimental vehicles have followed the new sequence. However, others have not, most notably the Stryker family of wheeled fighting vehicles which have been designated between M1126 and M1135. Vehiclces of the cancelled Future Combat System program were given designations between XM1201 and XM1219.
* Author's note: the exact date for the change in designation system is unknown to me and any information is appreciated. The M60 tank is the first to have used an XM- rather than a T-number during its development which began in 1957, and it is also the first to have been given an item name with multiple modifiers (Tank, Combat, Full-tracked, M60). A year earlier, the T43 tank was renamed M103 which may be another clue that the change happened sometime in 1956-57.