I think [MENTION=17926]unworthy1[/MENTION] nailed it.

That bottom bracket shell certainly looks like it came out of the Losa shop. The "M" could (and I'm not 100% sure here) stand for "Magni" as Losa did build Pep Magni frames.

The OP's frame here could be something along the lines of this Pep Magni made out of Dedacciai tubing (you'll notice some differences but also quite a few similarities):








You could check the dropouts and see if there is a Losa pantograph there, underneath the paint.