For these ignions, they are all the magnetic inductive pick-up type which triggers the coil from a distributor signal, four or six cylinders, it's still a single discharge, unlike the later electronic ignitions which are specifically discharging for a six or four spark plug engine. Any should work, as long as the B21F has it's correct distributor and coil. It's probably easier to find a 240 ignition unit anyway, so you should be able to find one that works.