there were a lot of different types of bulbs -- the most common sold for consumer use were made to be used at "M" synch setting, which fires the bulb but delays the shutter a titch until the bulb has reached its brightest point and can be used at all shutter speeds on a rolleiflex or any camera with that type of shutter. "X" synch for strobe fires the bulb the instant that the shutter is wide open because with a strobe/electronic flash there is no delay.

M2B bulbs are the same as M2 bulbs only blue to balance the light for color film. M3 bulbs, looking at some auctions on ebay, look similar to M2, probably have a different flash power.

Flash bulbs were a lot brighter than strobe flash and allow for a smaller lens opening, although I suspect some of that is because the bulb burns longer than a strobe flash.

There was also MF bulbs, which were designed for use with focal plane shutter cameras and could synch at all shutter speeds because they burned longer. Most bulbs, like those m2 ones, had to be used at a slow shutter speed with a focal plane shutter.

certainly no shortage of bulbs for sale