Fishbreath and I often like to laugh at the Russians and the goofy naming conventions that they have for things that have been produced a lot, like the T-55, the T-72, or the Su-27. That said, they’re not the only ones to do it, and the naming schemes for my favorite tank, the M1 Abrams, are particularly annoying.
Here they are, in order (ish) of when they were first made.

XM1
XM1¹
M1
M1IP²
M1A1
M1A1HA³
M1A1HA+⁴
M1A1HC⁵
M1A1D⁶
M1A1AIM v.1⁷
M1A1AIM v.2⁸
M1A1SA⁹
M1A1FEP¹⁰
M1A1KVT¹¹
M1A1M¹²
M1A1SA¹³
M1A2
M1A2SEP¹⁴
M1A2SEP v.2¹⁵
M1A2SEP v.2 TUSK¹⁶
M1A2SEP v.2 TUSK II¹⁷
M1 TTB¹⁸
CATTB¹⁹
M1²⁰
M104²¹
M1²²
M1²³
M1²⁴

1. There were two rather different prospective designs for the M1, one made by GM and one made by Chrysler. Both designated XM1 for the trials to see which was best. Figuring which XM1 is which is an exercise left to the reader, who will no doubt find it enlightening.
2. IP for ImProved.
3. -HA for Heavy Armor, i.e. it’s got first generation depleted uranium armor.
4. M1A1HA subsequently upgraded with second generation depleted uranium armor. Unofficial, but included for completeness.
5. -HC for Heavy Common, i.e. made with second generation depleted uranium armor.
6. -D for Digital. An M1A1HC upgraded with electronics to the M1A2 standard. No factory reconditioning is expressed or implied by this designation.
7. AIM for Abrams Integrated Management. Earlier model factory reconditioned to be like-new, and then a whole bunch of electronics are added.
8. As -A1AIM v.1, but also upgraded with third generation depleted uranium armor.
9. -SA for Situational Awareness. As -A1AIM v.2, but with a confusing new name. See -A1AIM v.2.
10. -FEP for Firepower Enhancement package. As -A1SA, but for the USMC. See -A1SA.
11. -KVT for Krasnovian Variant Tank. M1A1 modified to look like Soviet tank for training purposes.
12. Export variant for Iraq. Because they got their butt kicked twice by Abrams tanks, so they figured they might as well buy some.
13. SA for Special Armor. Export variant for Morocco. Not to be confused with the other M1A1SA.
14. -SEP for System Enhancement Package. Adds third generation depleted uranium armor and some improved electrical systems.
15. Even better electrics.
16. TUSK for Tank Urban Survival Kit. Makes the tank better suited for urban warfare. Technically, may be applied to any variant. Usually seen applied to A2, A2SEP, and A2SEP v.2s.
17. TUSK II has better supplemental armor than TUSK.
18. Tank Test Bed. M1 variant used for a bunch of late 80s experiments.
19. Component Advanced Technology Test Bed. A more-radical, experimental, late-80s tank.
20. The Grizzly Combat Mobility Vehicle. No, I can’t use the name again because that’s not the designation. I didn’t write “Abrams” after every designation up there, did I?
21. Wolverine assault bridge.
22. Panther II Mine Clearing Blade/Roller System.
23. The Assault Breacher Vehicle²⁵.
24. Armored Recovery Vehicle. No, roles do not count as part of the designation, or else I would have written “MBT” after a whole bunch of things up there.
25. What’s the difference between Combat Mobility Vehicle and Assault Breacher Vehicle? No idea.