1974 Brain Box saves the day.

The collector/heatsink is electrically hot in the circuit, it connects pin 2 to 3 and pin 2 is also the heat sink portion. The voltage is only 12V, not enough to shock you (unless its a flyback from the coil) if you are going to attach it to the heatsink directly, you will short the collector (+) to ground (-) via the heatsink unless its isolated from the case, probably not! The 'tape' you see may be mica, plastic or another insulator but your screw needs to be insulated as well from ground with a bushing, or use a nylon screw. Best bet is to allow the heatsink to be isolated from the case (on plastic isolators) and then you can run the metal screw into the collector/sink and use it as a conduit for the heat. It would work better if you actually cut a hole in the case and allowed the collector of the TO-220 to bend out to contact the isolated heatsink.

The pins 2(+4, the heatsink) and 3 are the open switch, when a small current is applied to the base (1) it allows the switch to close allowing current to flow between 2 and 3. remember 4 (heatsink) is also pin 2, they are internally strapped. The small current is supplied via the reluctor and its associated circuitry. This acts as a high speed relay, switching coil on/off up to 12,000 RPM (800 times per second) or 800Hz with a very small trigger voltage.