Hmmmmm- - - - -there's supposed to be a tab that prevents the square plug from getting reversed. Sometimes it gets worn away or broken off. It's been so long since I worked on the external-regulator rigs that I'll need to dig out a wiring diagram to answer your question about the extra terminal. I think it might be a connection for an external ground wire. No, it won't be possible to substitute an ammeter for the idiot light, as there was a different circuit and a shunt-type ammeter for the trucks that had the ammeter option. There was also a resistor built into the ammeter-equipped trucks to prevent enough current backing up through the circuit to keep the ignition working as the switch was shut off. The idiot light restricted the current enough to prevent the run-on situation. It would be possible to wire an aftermarket ammeter into the main charging circuit between the battery cable terminal at the starter and the alternator. Other locations would not allow the proper charge/discharge indication at the ammeter.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!