you are correct sir
yes, TPS required the 3 prong plug to open and close the solenoid. Hence why our '70 carbs don't have any ported vacuum openings.
Yes the other one is typically plugged, although some folks on this site have indicated that that's where they stick their real sensor so they can have their broken 3 prong sensor sticking out front looking all original. Rather clever idea that i wish i had thought of before I tossed my 3 prong plug.
Aaron
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