This frame looks like the work of Mario Peloso (Alessandria). Even though I don't have any pics of an exact match, I'd be surprised if it didn't come out of his shop.

Peloso built custom frames and generally would have initials of the customer (or another shop he built for - see the Suzzi frame below) stamped on the seat lug in the format seen on your frame.
Sometimes full first or last names would be stamped, some had "Peloso" and some had words like "Special" (not sure if those indicate some sort of model).
Perhaps on your frame the word was filled in with brass because the original order didn't go through as planned? Who knows.

Peloso head badges were attached with two horizontal rivets. The little port on the underside of the bottom bracket shell was also common on Peloso frames of the era.

You can find a bit more info on Peloso in this post by Mike Barry,
and another one that dovetails with it from an Italian who knew Peloso back in the day (it's in Italian so you'll have to use Google translate - just remember that any time you see the word "hairy" it's actually "Peloso" ).

Here are some examples, starting with one that has similar seat stay treatment (which wasn't common on Peloso frames) as well as the long chainstay crimp:











Suzzi by Peloso: