I think the timing disc is supplied to manufacturers as an accessory to help them get the knobs in the right place on the rotor while developing. Once you have the pulses in the right place relative to the crank position, the small adjustment needed to get the pickup positioned correctly can be read off the igniter and adjusted accordingly.
I can't fathom out why later igniters don't work with earlier bikes, that's different from the 360's where they seem to be interchangeable. From workshop manuals I have, wiring diagrams suggest the pinouts haven't changed, unless someone knows different. Or maybe later igniters had a different firmware version.
