I tested a C1 SMG magazine in a Mk4 Sterling a couple of years ago. It functioned perfectly. The breech block and trigger en bloc also worked.

FAMAE PAF magazines and trigger en bloc work in the Mk4 also. The breech block needed a little work. The cocking handle hole is not in the same location and had to be moved to work in the Mk4 and Mk5.

The FAMAE PAF breech block was made with the same off center internal lightening cut Mk5 breech blocks have. It uses a single return spring with a higher rate than the Mk4 big spring, so the cyclic rate is a little higher.