Olympus, Nikon/Ricoh, Minolta, Canon/Pentax flash control protocols on a multi-system flash made by a 3rd party. The switch activates/deactivates the appropriate contacts for the different cameras that it could be used with.
M could also be Manual...hard to know without seeing the entire back of the flash, but I suspect it is Minolta, and not manual.
I am not sure what St is for....perhaps Stroboscopic, with center-contact-only is my best guess. Repeating, or stroboscopic flash was kind of a "thing" back then.