What is this?

That is the solenoid that directs the vaccum signal to the distributor's vaccum can. On the later (post '72 IIRC) cars, there was a timing resistor that openned the choke, and delayed the vaccum to the distributor's vaccum can until the engine temp was up. (it only used time, not temp, but it was set for like 5 minutes of run time) When the resistor closed the circuit, that solenoid openned, and vaccum could get to the can. At least that's the way I remember it.