Car early type electronic rev counter

[Jonathan Robson]

The first electronic rev counters from the sixties usually connected in series with the contact breaker points, they normally had male and female bullet connectors for this and a Lucar connector for the power. The 'IMP' on the back of yours signifies the later impulse type, the 3rd terminal you refer to would is probably marked I rather than 1 and should connect to the 'switched' side of the contact breaker points, the other two are for 12V power as marked.