ORIGINAL: The_Stuff
the top your using there is the same one as buddy has. noted the "rim" looking thing on top. most visible in the picture close to the commander dude.
M1A1 had a blank plate, often covered by a spare wheel, M1A2 has the CITV (Commanders Independant Thermal Viewer)
The big bulge on the rear corner is the APU (Auxillary Power Unit) - I hate them in this position, and usually put them in the bustle rack (more often found there)