The S does in fact have a removable back like the later S2 and S2a, I must have been thinking of the C. Fortunately this makes things easier, remove the back by pushing in on the darkslide and pulling it off. You can now wind and fire the shutter, if the speeds seem good and the aperture is stopping down you should be good aside from possible light leaks. You cannot wind and fire the shutter with the back attached and no film loaded, the advance knob will just spin freely. My first S2 had a light leak through the dark slide slot, I was able to disassemble the back and replace the spring loaded seal. My second S2 was leak free.