I just replaced my 127 Ektar with a 135 Optar in a Graphex shutter.

I know that the black letter M and the red letter M are for settings when using certain shutter speeds with flashbulbs.

However, there is a F.X setting, the F is red and X is black. I assume this is for electronic flash sync, but what does the red F stand for?

Also, there is an OFF position. What happens to X sync if OFF position is used? Does it still work with electronic flash in OFF position?

What is the reason for the OFF position and what effect does it have on sync speed or flash choice? Thank you.

I think the F is for SM type gas filled, short duration flash bulbs. Never tested the "off" position.

"Off" means just what it says: off. IOW, the synch gear train is disengaged, (theoretically) saving wear and tear on it. Also, there should be no contact for an electrical path through the shutter. If the synchronizer is out of adjustment, you may get contact in the "off" position, but it shouldn't otherwise affect the operation of the shutter. That's been my experience with Graphex shutters, at any rate.

Thank you gentlemen.

The F is for fast peak bulbs, i.e. the Speed Midget gas filled bulbs. It should provide roughly a 5 millisecond delay.