Thanks Jim,
Kodak used to provide inserts with their films like Kodacolor-X. They included guide numbers for common flash bulbs. All bulbs had Guide numbers for both X and M sync listed for various shutter speeds. However the M2B was only recommended at X sync at 1/30th sec. (M sync was listed at Not Recommended on the table). I always wondered about this. If the M2B peaks at only 13ms that explains it.

For those who donít know.
With a M Sync the camera sends power to the bulb and starts the burn then typically waits 20ms until the bulb is at full power before opening the shutter. That is because most Medium peak bulbs took 20ms to reach peak light output and 20ms later (40ms after pushing the button) the light is almost gone. If an M2B that peaked at 13ms were used at M sync then the shutter would not open until 7ms AFTER peak and over 80% of the light output would be gone before the shutter opened.