I've followed this and the thread on Shotgunworld http://www.shotgunworld.com/bbs/viewtopi...94f589fba73e54a
and have nothing smart to contribute but will do so anyway.

I see no Belgian proofmarks, specifically no Perron nor Provisional Proof.
No luck searching the internet for 'M over CM' or 'CLA'.
I think the gun is a modern reproduction, possibly Italian, but can't explain the absence of Italian proofmarks.