It is not always true that CETME builds (almost all CETME's in the US were built from parts kits using US manuf receivers but a tiny few were originally imported as sporters from Spain ) will use G3 magazine. And everytime I mention this some CETME experts will jump on me. But I owned a Century built CETME that would only accept the CETME magazines.

The magazine well on the original CETME was designed for a curved magazine. The G3 magazine is straight. Some of the lowers that Century used were designed to match the original CETME mag well - like the one that I owned. Others were closer to the G3 shape. So never assume that a CETME build.