The solienoid in the area you described is the TCS plug in. I remember that when I got my L78, and my friend got his 70Z, we were just settin in the car messin around with the shifter and noticed when you shifted it through the gears that it gained rpm(we checked both cars). We dug out the manuel, and it said it was called TCS.They got disconnected as we were told they worked against opening the secondaries in the carb.
I was pretty sure it was 1st to 2nd, but it could be 2nd to 3rd. I know it was not 3rd to 4th. I expect it still would work on my deuce if I hooked it back up.