Most metal double-stack mags are based off of the Browning Hi-Power mags. The angle of the grip/mag is the same, the magazine realease cut is the only thing that changes. The followers may be slightly different as well, but in general, if you change cut a new release catch, it would fit. This was done very frequently during the old AWB so that you could have new hi-caps at lower prices. Beretta 92 mags were cut and new base plates fitted for the Springfield XD, for instance.
It also makes mass-production easier. Mec-Gar makes most of the mags, so the make one tube and then alter the cuts. This explains why all of a sudden, a lot of the metal mags went from Beretta 92 smooth tube style to PX4 ribbed.