Quote Originally Posted by summicron1 View Post
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.
The blue (daylight balanced) bulbs are approx 1 stop less light than the clear bulb.

But remember that GN is also a function of reflector type. GN for M2B in a polished bowl is 120. In a flatter typre reflector it is a bit more than 80.

... and don't forget about efficiency compensation if flashing at speeds above 1/50 or so.