I just got a great batch of old stuff, and among the flashbulbs are several samples of a bulb I have never heard of. It is a Westinghouse SM, and it is the size and bayonet base of a 25. The really weird thing is that there is no flash filament in the bulb, only two large electrodes connected by a very fine filament. They are safety coated. The only thing I can guess is that they are gas filled, and rely on the filament to ignite the gas.

Am I right?

Yep they are gas filled have a 5 ms delay with a very short duration and not a whole lot of light, but this is what was used to freeze motion of skaters and acrobats. Sort of a disposable 285 vivtar.

There was another 5-millisecond delay gas-filled bulb, the "SF." These were what the "F" setting on an "MXF" leaf shutter was for -- most modern shutters in the 1950s offered those choices. Indeed, the gas-filled lamps do not put out a lot of light, but the illumination rises and falls very quickly, so more light gets through the shutter at 1/400 or 1/500 second. I used them a couple of times, and they do work as advertised.

SM (Speed Midget) bulbs = 3300 lumen seconds of light. Flash duration equal to 1/200 second shutter speed.
Able to stop motion if it isn't too fast. Good mostly for close up shots, 6 to 8 feet with medium speed or faster films. They don't get hot, just warm to the touch, and they turn white when fired. I have a whole case of the things but have no idea why.