The Zastava M51 is a prototype Yugoslav submachine gun developed before the adoption of the M56. It uses elements of the Beretta M38 (namely the captive recoil spring system) and PPSh-41 (the selector switch and magazine well) along with a collapsing stock reminiscent of the Polish PM63 machine pistol. Documentation on the M51 is very limited, and I don’t know many were actually built, but it never went into series production.
A big thanks to the Croatian Police Museum (Muzej Policije) in Zagreb for giving me access to film this rare piece for you! Check them out at: https://muzej-policije.gov.hr