The Government Model is the original version with a 5" barrel, 7 shot magazine, and a spur hammer.
The Commander has a 4 1/4" barrel, the rounded "burr" type hammer, and a shorter tang on the grip safety. The original version had an aluminum frame, later Combat Commanders had steel.
The Officer's ACP has a 3 1/2" barrel with a larger diameter muzzle, a shorter grip frame, a 6 shot magazine, a larger hole barrel bushing, a different recoil spring plug system, a double recoil spring, and a shorter mainspring housing.
The Defender is basically a slightly shorter Officer's Model with a 3" barrel, aluminum frame, and high-grade accessories such as a up-swept grip safety.