The standard 20 round Chinese mags usually work pretty good. It sounds like it was one of the Norinco shorties. They had altered or block lugs instead of the actual functioning bayonet lugs. Often these came in with a crappy thumbhole "Sporter" stock that was very blocky up front a well. An interesting variation, and most folks chucked those stocks.
I remember getting them in at the range/shop I worked at part time in the late 80's or early 90's. Back then they were right around $100 and a case of 762x39 ammo would run you another $99. They would come in as a "Cowboy", Paratrooper" or "Sport" model. That may be what the extra "S" is there to indicate.