Tom,
I replied to your other thread,
here. That's probably a starter solenoid. If it's wired between the coil and distributor, that sounds pretty weird. It shouldn't be like that. There shouldn't be anything between the ignition switch and coil and the coil and distributor except wire.
If you can post a pic of it, that would help us figure it out for you.
Here's a picture of the wiring diagram for 48-49 trucks showing the solenoid.
