I finally found the answer to the question that started this thread in some old printed GE information.
The M5 and M25 were the same size and had the same total light output. Their timing was different. The M25 was 15ms to peak and the M5 was 20ms to peak. The M5 was made to have the same 20ms to peak time that the larger and brighter GE #5 had. The larger GE #5 put out about 25% more light then the M5.
Simple box cameras of the time ( 1950s ) had fixed shutter speeds of about 1/30 sec. That is about 32ms long. They also had what was in effect X sync. There was no delay in opening the shutter on these simple and cheap cameras. They relied on long shutter speeds for flash sync. They also had small fixed F stops of about F16 to get maximum depth of field with their fixed focus lenses. The reason an M25 bulb was recommended for these simple cameras was all of the flashbulb’s light output would be during that 32ms. With a slower M5 bulb the shutter would close before the bulb burned all the way out. This would result in a reduced guide number for the M5 if used in these simple cameras.