Am I remembering correctly that there are two different types of Model B mags also?
I seem to recall one has a 'tab' that sticks out the front, and the magazine fits flush with the end of the grip frame, with that tab sticking out a small slot in the frame.
The other has a wide base that covers the entire bottom of the grip frame, and nothing goes into a tab on the frame.
That is my recollection. I could be wrong.