HI Everybody
I think I have at last solved the problem.
The Prestolite alternators are now overhauled by Electrosystems,Inc. and their blerb states that the AUX. terminal is for connection to an electronic tachometer.
This makes sense as I believe this is the system used by a number of vehicle manufacturers.
Just requires an electronic division circuit for pulley ratio and number of phases to output r.p.m.
Been an interesting exercise, thanks for everybodies input.
Regards