Italian builders that used fastback stays brazed to the pinchbolt include (later) Pogliaghi's and Freschi (who worked at Pogliaghi). But yes, it is uncommon for an Italian frame to have them.

That frame looks like late 1960's to 1970's English or American, maybe South American ? Maybe Belgian or Dutch ?. Some of the details look somewhat amateurish to my eye.
Those Campy "vertical " dropouts were crude in design, and they weren't "vertical". Most every frame I've ever seen with them has them positioned so the rear wheel has a harder time trying to be removed or installed.

It might be a home-made frame by an Amateur or a Machinist ?

The "M" cut-out in the BB shell is the only detail that makes the frame look like it might be built by an actual Framebuilder. Except that a machinist would have pretty easily been able to do that.

No braze-ons at all also makes me scratch my head.