It's a top gear switch, with spade connectors instead of the bullet connectors on the reverse switch. It's used with the vaccum advance system, so there is no advance in 1st, 2nd or 3rd, only in 4th gear. This is an emissions part & was introduced in 1976. There are also temp senders in the head & vaccum canisters on the inlet manifold, as they only wanted the system to work after the motor warmed up. Easiest fix is to run a vaccum line straight from the carby outlet to the distributor, because ADR27a has been superceded & is no longer enforced. Don't use the top gear switch as a reversing light switch, either, as the reverse lights will be constantly on, except when you put it in reverse.