As a recently retired Marine Corps tanker of 23 years and Master Gunner that started on the dinosaur M60A1 RISE/Passive then NETT'd to the M1A1, with deployments in Desert Storm and Iraq, to include being an embarked tanker on amphibious ships (discontinued after Desert Storm but restarted several years ago), I can tell you all you ever wanted to know about Marine Corps tanks, and a little on the M88A1 and M88A2 HERCULES. I have a close friend in 'stan now working on repairing the Abrams and Hercules in country, a retired 2146 (tank mechanic) Gunny.
I noticed some mention "old" APU (Army hull mounted) mounts but these were never present on any Marine Corps depleted uranium armored M1A1's. In the mid 1990's the Marine Corps did acquire some Army surplus non-Heavy Armor M1A1's but I don't recall seeing these with hull mounts for APU's. These "plain jane" M1A1's were only used in training as the original allotment of factory built USMC DU M1A1's were reserved for war stocks and on pre-positioned ships. Eventually the Marine Corps purchased additional Army surplus M1A1's due to Iraq but these were the depleted uranium armored (or Heavy Armor) versions then rebuilt by GD to Marine Corps specifications. All plain jane M1's have since been withdrawn from all units and a few completely remanufactured as the Marine Corps Assault Breacher Vehicle (ABV).
Also, the MCD was discontinued and removed from the Marine Corp's M1A1 SL-3 long before Afghanistan, though GD continued to put the MCD mounting rails on the turret ITV cover on complete rebuilds, but it appears they finally deleted them as they are no longer present on any USMC tanks in Afghanistan.
Regarding Marine "markings" on USMC tanks, the only "factory" markings are the "USMC serial number" on the upper rear corners of the hull, and "LIFT HERE" next to various lifting eyes plus a couple of small instructional markings such as on the EAPU. All other markings are unit and crew added, such as company chevron and platoon hashes, which is at the discretion of the company. Since Marine combat deployments are typically 7 month rotations and tanks are very expensive to transport, the equipment is RIP'ed to the next unit which in turn may simply paint over previous unit markings. Due to the rigors of constant, long combat patrols, tanks get worn relatively quickly on deployment, so newly rebuilt tanks are rotated in typically every other RIP. BTW, there are several extra M1A1's on hand in country that serve as replacements in the event of combat losses.
Anyways, I'm happy to provide technical and historical assistance to anyone if needed!