The HS-91 is a Croatian-made submachine gun form the Homeland War period of the early 1990s. This is the only example known, and it was seized well after the war by the Croatian Police (and currently resides in the Police Museum in Zagreb). Nothing is known about the details of its origin or production.
Mechanically, the HS-91 is a hybrid of Yugoslav M56 and Soviet PPSh-41. The barrel, magazine, and magazine well are from the M56, while the bolt, safety, receiver construction, and firing system are from the PPSh-41. It is chambered for 7.62x25mm Tokarev, and uses standard M56 magazines.
A big thanks to the Croatian Police Museum (Muzej Policije) in Zagreb for giving me access to film this rare piece for you!