These guns aren't modern cartridge guns like S&W, Ruger, et al with drop in parts. Early on they were made in small Northern Italian shops, tolerances were vastly different, even though at first glance they all looked alike. There was even a consortium in the 70's-80's named C.O.M., wherein the small shops would manufacture different parts and C.O.M. would assemble them under the C.O.M. brand so as none of the small manufacturers would be held liable for inferior parts, and C.O.M. would bear the brunt of any imperfections. Needless to say, C.O.M. only lasted a few years.
Before Pietta went to CNC machining ~2002, they subcontracted parts from small shops and then hand-fitted them to produce a marketable firearm. That is why Pietta pre CNC guns are hard to find parts that fit.
Regards,
Jim