Camera gun film cartridge??

Found something interesting the other day, didn’t buy it as am not sure what I would do with it. I cant post pictures of it because they are on my cell phone so a description will have to do till I get the pics to the computer. Anyhow, its a small box, about 6″ square, 1.5″ deep looks to be made of aluminum. Has a key built in for winding a spring and what looks to be camera lense on one end. The only nomenclature on it is “For use with gun camera. Mark VI Mod.3 & Mark VII Mod 1.” Located near the nomenclature is a slide switch, marked “S” in one position and “M” in the other position. I was thinking film cartridge but why the lense? I tried to open it but its not obvious how its done. Any thought on what it is and what it fits?