I have noticed there are a few differences between magazines, here is a pic of two of mine showing different base plates.



and the seams of the same two mags.



Any comments on the identity of these base plates, seems there could be a whole line of research just on this one component

The flat one is the first or early one, the indented one is what they call the Type 2, a later development on the 15's. Here's two of mine. Mike