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
“(…)collapsing stock reminiscent of the Polish PM63 machine pistol(…)”
Was there any earlier design using similar solution which might inspire M51 designer?
What are advantages of this solution as opposed to under-folding stock as found at ERMA EMP 36, see 1st photo from top http://udachnyj-enot.com.ua/pistolet-pulemet-erma-emp-36-za-polshaga-do-mp-38-40/
Good question not sure if its advantage or not but HK has been using it a long time (G3 33) and still with a MP5 version. Lack of any hinge mechanism and only a locking one maybe?