Howdy Nick......by your reply I can see that you know already most of
what I mentioned I don't recall power mosfets back that far but it's
possible. Wouldn't rule out that the device, especially if it's a
TO-220 or similar package isn't an SCR or similar either. I'll draw up
a schematic of an era CD box I unpotted and repaired for a Can-Am
motorcycle which is right at hand and shoot it off to you. The RC
circuit can basically be found in any basic electronics text book with
the formulas for finding instantaneous charged voltage etc....some
experimentation will be needed to find your offset though. There were
quite a few CD ignition kits around at one time and articles in various
electronic mags and project books so some digging there might be real
helpful.

Gord