Are you reading from the manual or does your camera actually function like that?
I know previous versions, like the 690BL would work like that, but the three GL690 owners I know (including myself) have verified that the R/S switch is not operational (doesn't matter where it points).
They all seem to work "automatically" - in other words, when film is in the camera the frame counter works and you need to advance the film a full frame to release the shutter (two strokes).
Without film you can dry-fire the camera with just one stroke (or expose a piece of sheet film if you wish).
No need to flip the R/S switch.
I don't know if this is valid for all or just part of the GL690 production.
I have a GL690. The R/S switch is functional, and the camera won't dry-fire on "R." I had not previously tested what happens if you have a roll of film loaded and it's on "S," because of not wanting to waste film, but I just tested it with a roll of backing paper. The "S" still defeats the shutter interlock, so the camera will let you fire the shutter after the first wind stroke cocks the shutter.
I think the R/S switch uses a spring to pull a lever that interlocks with the shutter release, perhaps yours has a broken or unhooked spring? Maybe repair-people bypassed it deliberately. But then it's not clear to me why you can dry-fire your camera on "R."
A sort of table for my camera:
- no film, switch on "R": can't dry-fire shutter.
- no film, switch on "S": can dry-fire shutter after one stroke of wind lever.
- film/paper, switch on "R": counter advances, 2 strokes per frame, shutter cocks on first stroke but can't fire until second.
- film/paper, switch on "S": counter advances, 2 strokes per frame, shutter cocks on first stroke,
can fire after first stroke.
I recommend being cautious with the film + switch on "S." If you happen to fire the shutter after the first stroke, you still need to make a second stroke to finish winding the next frame. If this stroke finishes before going all the way around, then it won't release the interlock and the camera will appear to be jammed. The cure is to pop the back open and finish the extra little bit of wind stroke. Not something one would want to encounter in practice.