It looks like the M3 bulb came out in 1963
This information in an April 1963 Article in Popular Science.
“Westinghouse is introducing ...The M3 will take the place of the No. 6, M5 and M25 bulbs...Zirconium filling in the new bulb comes to a high intensity level (twice the output of aluminum) quickly and maintains it for a long time.... It can be used for most focal-plane shutters, all leaf-type shutters”
Now if anyone had a graph of the light output of the M3 that they could post that would be great. We could compare it to the 1957 Sylvania dealer sheets I posted we would have a good idea of how it stacked up to the FP-26 bulb (quite similar to the the GE No. 6).