If you choose to use the bulbs, be VERY careful when removing them.
They get quite hot, and should be left alone for a couple of minutes to cool.
Originally there were three categories of flashbulbs, Slow, Medium, and Fast, abbreviated S, M, and F on the shutter.
The S and F bulbs fell out of favor, leaving M as the only choice, and the only flashbulb setting on later shutters.
The X setting is for instantaneous flash (electronic strobes).
You may find a letter V on some shutters, at one position of the sync lever.
This sets the self-timer and has nothing to do with flash.