I replaced the factory started solenoid on my '66 Belv with one from the local parts store, and my starter would not engage. I noticed that the factory only has 3 connections, but the replacement has 4 connections. The bottom right connection on the replacement is not on the factory one. What do I connect to this post?

Thanks!

ground it w a short piece of wire w a 1/4" female brass terminal on one end NAPA 725145 and a ring terminal (eyelet) on the other end of the proper gauge/ID 2 fit/go under the relay mounting screw w the orange paint on it.

wouldn't that terminal be the wire from the neutral safety switch

yes on a 4 terminal app but if you ground it permanently it'll function just like your OE one that has that 4th terminal grounded to the case if you look at it, it's not a male straight up one but is flat & connected 2 the relay case. Your OE nss safety protection is located somewhere else iirc the clutch pedal or side of trans

For future reference it is a starter relay on the fender and the solenoid in attached to the starter itself.

Thanks for the help! I really appreciate it. I was going crazy trying to figure out why it wouldn't engage.