The “M” position is “Medium Synchronization”. The M setting applied electricity to a wire filled flash bulb 20 milliseconds before the shutter reached full open. This 20 millisecond delay was necessary to allow these old fashion flash bulbs to heat up and then blitz. So don’t use the “M” setting unless you have flash bulbs. If you use “M” with electronic flash, the flash will fire before the shutter opens.
The “X” position is “no delay”. In “X” setting the shutter opens, upon reaching maximum wide open, the electricity is applied to the flash and it fires immediately. This is the setting you should use for your studio and all other electronic flash units. The “X” synchronizes at every (all) shutter speeds.Because the same leaver (switch) was used for the self-timer and the synchronization setting, setting the leaver to “V” allowed time for the photographer to get into the picture a “selfie”.
Once set to “V” at the end of the allotted time, the shutter fired however the synchronization is set to “X”. This wasn’t good if you were using old fashioned flash bulbs because they need the 20 millisecond delay provided by the "M" setting. Because the flash bulb continues to output for a time, a slow shutter like 1/30 of a second will allow synchronization. However the warning is moot for electronic flash as it always works in the “X” setting.