Actually the P25 and P26 from Sylvania and the GE #5 and #6 were made together for many years. The P26 and # 6 did have a much flatter peak. This allowed their use with focal plane shutters. Times were published for speeds up to 1/750 of a second on the packages I have. I believe that the M3 bulbs also has a flatter and longer peak similar to the P26 and #6 bulbs. In addition the GE #6 and FP26 were larger bulbs with larger bases just like the GE#5 and Press25.
You may be correct in that the M in M3 and M5 may indicate medium peak and not mini or midget bubs as I had thought.