If the magazine body is completely smooth, without any locking bumps on either side, but the plastic bottom block is long for a 22/45, then it is a Mark II magazine metal body mounted onto that bottom block. The metal magazine bodies are externally identical dimensions. What sets them apart for the model is the locking bump mid-way down the right side for latching the magazine in the frame with the thumb button. All Mark III magazine's have this same feature in the side of the mag body because they use the same method of latching the mag.
R,
Bullseye