The magazines can be interchanged. In fact, the Dutch manufacturer made such a mess of their first attempts that Mauser actually sent them a functioning Haenel magazine to use as a reference
For a short period of time, Interarms had Mec-Gar magazines with their name and a Mauser-like logo on the spine. It is also not uncommon to see Interarms Parabellums NIB with the magazines swapped for Mec-Gars.