The U.S. Army and Marines both use the same M1A1 tank. But each service equips them in a slightly different fashion. So it is possible, if you look carefully, to tell an army M1A1 from a marine one. The differences, however, are subtle. For example, the smoke grenade launchers used by the marines are a different model, with the launch tubes spread out a bit more than on the army launcher. The marine tanks have a fording kit on their tanks (usually) and a different arrangement of the shackle points (used to help get the tanks onto amphibious ships.) The marines also use a different GPS receiver for their vehicle location system.  The outer steel on the marine tanks has a different finish, which represents anti-corrosion treatment to deal with the salt water these tanks are often exposed to.