Here is a photo of the guide number info. on an old* box of GE M2 bulbs. See the note about the reduced guide number when using an adapter for this small bulb when used in a flash made for the larger No. 5 or Press 25 bulbs. For those not skilled at guide number calculations; you loose about one stop of light with an adapter.
*Back before they changed the film speed rating system and Tri-X was only rated at 200. If anyone knows when they revised the system by how they set film speed and any details please let me know. If nothing else it will help me date this box of flashbulbs.