1974 Brain Box saves the day.

Let me back up a little: We know the case is the ground path. We know the heatsink of transitor is pin 2, the collector. What the transistor does as a switch is to bridge (or make resistance very low) the collector to the emitter via a trigger at the base. In a coil igniton, we are bridging the ground path from the coil negative to ground (which is just the other side of the positive coil terminal). There is something called back-emf, that needs to be quelled to prevent transistor damage by a few diodes wired head to head. We need to get @KitCarlson to chime in on this theory of operation. To me, it sounds like the case has always been electrically active as the ground path, but the heatsink is isolated from the heatsink /collector of the transistor itself for some reason. Maybe the earlier ones had painted cases for insulation from touching them but its still a ground, you can sit all day with your hands on the ground terminal of the battery and not feel anything. Perhaps we should just focus on the heatsink aspect and not try and re-engineer the operation of this.