Semi Sterling parts

Found some interesting Sterling semi-auto parts. The breech block face is cut differently than any I've seen so far. It's also a different return spring and firing pin retainer. Looks like a single return spring system. Wondering if Captain Laidler might have more information??

Attachment 43569
Attachment 43570
Attachment 43571
Attachment 43572
Attachment 43573

Put it in a Mk6 and it worked. :D Bit hard to charge though.

Going to try it with 7.62x25mm next.