This function depends on a couple small tabs on the dark slide that actuate an interlock on the body. If you use a dark slide from another roll film holder, those tabs won't depress the interlock and the camera will still fire. Likewise if you use a roll film back for another 2x3 Graflok (like a Graflex 22, 23, or RH10). Also, if the dark slide is backed out just 2-3 millimeters, the interlock will be released -- but the film won't see light from the lens.