Looking further into it, GE must have changed the classification of the M5s at some point. I have a box of M5s that mention that they are class M and will only work with M-sync (or a slow shutter). I also found a newer box of 6Bs that recommend AG-1B, M3B, or M5B to replace the bayonet-base bulbs, meaning at some point the M5 must have become a brighter version of the M3.