Rather than focusing on what it isn't, what's the answer to what it is?
Is it a pre-production prototype of an Italian replica? Or is it a one of kind U.S. produced prototype?
If we had an understanding of just how old it really is, that could lead to answering other questions, like what's the metalurgy?
The round cylinder stops look pretty unique, the trigger guard and grip frame look to be unconventional.
Maybe someone just tried to build their own replica operating from known blueprints or something like that?
It must have still taken a fair amount of talent and effort to build.
And from the look of the screws, it does look to be older than ~60+ years.
It doesn't look quite like a later production ASM, does it?
I'll upload the auction pictures for the purpose of future discussion since the auction link disappears after 90 days.