Most of the time the other side is simply the door to put film in the camera.  Last time I had one of these in my hands, at least, that was what it was on that model.  It wasn't long  that the earlier models who used internal reels went to the cartridge reel as it shows in the picture, and a modified canister reel that slid into a set of guides on the back of the camera.  Interestingly enough, some of those old rear canister cameras relied on the weight of the film canister to offset the weight of the lens assembly.