Quote Originally Posted by Jeff Searust View Post
I have done the same thing -- X is for Excellent, M is for Moron--- and this switch is found on a LOT of lenses. I know Mamiya all have it.
The word excellent starts with an "E", not an "X". It sounds like a Wollensack shutter with the delay for synchronization of flash bulbs.

The "F-X" setting is for "Fast" peak flash bulbs or for electronic flash. In this setting, the flash contact is closed as soon as the shutter is open.
The "M" settings delay the shutter from opening after the flash contact is closed and they are meant to synchronize "Medium" peak flash bulbs.