There are actually more than two variations of the Hammer.
Hammer P, standard right feed, plastic frame (hence the P), M16 grip, silver body.
Hammer A, standard right feed, aluminum frame (hence the A), M16 grip, silver body.
Hammer Plus, power feed (balls make 90 degree turn before entering the gun, hopper to the left side of the gun), aluminum frame, M16 grip, green body (I think).
New Hammer, power feed, plastic frame, 45 grip, silver body (I think, other colors may have been available).
There's also a Hammer 2, which is similar in design to a Sterling pump, and a Hammer 3, which was an in-line blowback semiautomatic.