The seller sheet I posted pre-dates the M-25B and M3.
I have a box of:
Sylvania NEW! Zirconium-Filled M-25B flash bulbs.
These must have come out in the late 1950s after this publication.
The M-25B has the small S.C Min. base that the M-2 has.
The box states that the total light output is 8,500 Lumen Seconds for the M-25B.
It also states "Set Synchronizer for 15 milliseconds"
This is fairly close to the Press 25B with a total output of 9,000 Lumen Seconds.
I assume that the M-25B is similar to the GE M5B.