The AMT M1 is the re-boxed Esci M1 kit. It was a poor kit even when Tamiya's old M1 was the only other Abrams kit on the market. When completed, it looks like an Abrams tank. At the same time, Esci/AMT released the M1A1 which was the first upgraded variant available in kit form. They took a lot of shortcuts and that kit is even worse as an M1A1 than it was as an M1. All they did was add an incorrect bustle rack, manhole cover, 120mm gun and correct ammo blast panels. They did nothing to the turret size and hull fittings.
The kit had link and length tracks, which was a first in the modern armor realm. Too bad the center guides were placed in the wrong spot on the interior of the track.
The M1 had a 105mm gun, short turret sponson boxes and no bustle rack. The ammo blast panels were three asymetrical shapes.
The M1 was upgraded due to feedback from the field. This resulted in the M1IP or M1 Improved Production model. It added the rear turret bustle rack and several other modifications like the cut in the last side skirt to allow mud to escape. Most, if not all, M1s were upgraded to M1IP standards.
The M1E1 was an M1IP that was given addtiional turret armor and a 120mm gun. This was never fielded, it was an experimental version used to proof the concept paving the way for the M1A1.
The M1A1 is the most common variant. All of them were armed with the 120mm gun and the ammo blast panels were two identical rectangles. There are several variants of M1A1s in use. Also a round panel was added forward of the loaders hatch. This was in anticipation of the development of the CIT-V. The round panel is referred to as the "manhole cover". An NBC system was added in place of the left hull sponson box. Turret armor was increased noticeably in size.
The early M1A1s used T-156 tracks that looked like angled football shapes. The majority of the M1A1s that fought in Desert Storm were of this type. The Army lent the USMC a battalion's worth of these tanks to use in Desert Storm as well.
Late M1A1s added depleted uranium mesh armor covered by armored steel to the forward slopes of the turret and hull. Various changes were made to make this tank suitable for the USMC (fording ability, etc.). Since this version was uparmored and now used by both the US Army and USMC, it was referred to as the "Heavy Common". Heavy meaning uparmored and common meaning it was common to both USA/USMC. Model companies call this one the M1A1-HA for Heavy Armor, but is really still just an M1A1. The track type also changed to replaceable squarish track pads. This variant is the one that fought the lion's share of the current war in Iraq.
Next comes the M1A2. It adds the CIT-V or commanders independent thermal viewer forward of the loader's hatch. It allows the commander to pre-designate targets while the gunner is engaging another vehicle. The cupola was changed as well. Most changes are internal using a digitized fire control and command and control system. The bustle rack mounted auxillary power unit makes its appearance in this version and is eventually retrofitted to early Abrams tanks. This is the type represented by the Tamiya, Trumpeter and DML kits.
Some M1A1s are upgraded with the same digital fire control and command and control systems. This results in the M1A1-D.
The M1A2 gets upgraded while in production and the M1A2SEP or system enhancement package results. This adds mainly internal differences and one of the main external differences is the addition of the APU added to the interior of the left rear of the hull in place of one of the fuel cells. The results of this deletes the fuel cap, and adds access panels as well as an exhaust nest to the left taillight. This is the version that saw combat in Iraq after the fall of Baghdad.