DD:
I went thru my old DELCO library and can find no hints at a classic failure method of the solenoid that would involve the auxillary ignition feed terminal from the starter's solenoid. Physically and electrically, that terminal is isolated from the rest of the assembly and by a significant distance. The only connection it makes is with the high current disk - same one connecting the "big" lugs - once the plunger is drawn into the coil.
BTW, most of the lit I have identifies this as the "R" terminal. I believe some of the newer parts (made years after my docs were scanned) may show this as "I" or "IGN".
The only failure mechanism I can see would be for a very high current event to cause the disk to "splatter" with the end product forming a partially conductive surface between the R terminal and the case ground. With the R terminal connected to the coil's "+", this could cause significant current to flow to ground, thru the ignition switch, with the key on. Given the age, I'd say the current draw from the 'splatter short' induces enough of a voltage drop to keep the main solenoid from engaging. Removing the connection at the "R" terminal should isolate the fault if inside the solenoid, assuming the key switch hasn't suffered damage.
based on that, I'd separate the engine from the harness and then assess the state of the starter & solenoid. If found acceptable, the switch is the next suspect.
