I once ordered a "B" cam for Minerva & discovered she already had one! (STUPID!) To identify which is which, look at the sides of the cam, the "A" cam has 8 distinct vertical lines, the "B" has 4 vertical lines.
The "B" cam has an increased dwell angle (the amount of time that the points stay closed) to increase the saturation time for the coil, so's you have a HOTTER spark, that'll CURL YO' HAIR or cause you to HURT yo'self!
I'm sure that Mike K. could explain it to us better, in SCIENTIFIC TERMS. If I had FORMAL TRAINING, I'd probably be an inventor of DANGEROUS THINGS!!
Bill W.