SandWatcher

I think you might be right about it being 9mm. The threads on the barrel aren't pristine, so tapping it up to a 10mm might improve...

10-1.25 metric seems too large. See comparison to the original in the center.

I'm replacing a corroded powder drum on a .45 rifle barrel, manufactured for a kit in the 1960s or 70s. The only markings are "Spain"...