The Sylvania Press 25 (P25) and the GE #5 were out it the 1950s, maybe earlier.They were similar flashbulbs in both size and light output but with different names. Both were filled with Aluminum and were roughly the size of eggs.

In the later 1950s they came out with the grape sized M25 by Sylvania and M5 by GE flashbulbs. The M denoted that both the bulb and metal base was mini or midget. These bulbs were filled with Zirconium and produce about the same amount of light as the much larger P25 and #5 flashbulbs. Both were advertized to reach peak brightness fast and hold that output for a long time. That is a long time by flash bulb standards. By the 1970s or earlier it seems that all the M25 and M5 bulbs were gone and replaced by M3 bulbs by Sylvania and GE.

My question is was there really a difference between the older M25 and M5 bulbs and the newer M3 bulbs or was this just a name change to help novice photographers figure out what bulb to use. The published guide numbers for the M25 and M5 are very similar to the M3 guide numbers.

Was there perhaps a difference in the time to peak. I have an M5 package that lists 20 milliseconds to peak and a later M3 that lists 17.5 milliseconds. Perhaps the older time was just rounded off to 20 ms.