X-sync is for electronic flash - the flash fires when the shutter is fully open. M-sync is for flashbulbs - the flash fires before the shutter is open so that it reaches maximum light intensity when the shutter is fully open.

The 5" reflector is for 5 and 25 flash bulbs; the 7" reflector is for 8 and 11 bulbs (IIRC) - it's been years (close to 55) since I used flashbulbs with my 23 Speed Graphic.

I've looked at flashbulbs on ebay a few times out of curiosity, and they were expensive.