That is "Film Pack". Film Pack is roll film cut into sheets and packaged in a metal cartridge with a backing paper for each sheet. Film Pack was one of the original ways to carry more shots in a compact format. Originally the packs had twelve sheets of film packaged in a way that was roughly the same size as one double sided film holder. Later they were increased to sixteen sheet packs. There were also other formats available. The packs for 2-1/4 x 3-1/4 were "520" and the packs for 3-1/4 x 4-1/4 were "518" and the 4x5 size was "523". I have seen Film Packs in Tri-X, Plus-X, Super-XX, Verichrome and Verichrome Pan.