Interesting! Is it in a harness/wire loom or is it running loose? from the location near the thermostat housing I'd almost say it's a temp sensor,do you have guages or idiot lights? Take your air filter housing off and take a couple pics of the carb,choke is on the passenger side,it
should be round & black,could be a heat riser/vacume set up,small steel tube oe tubes going to it,or it could be electric,one wire plugged on to it,on the driver/throttle side there should be an electric selenoid that makes contact with the throttle lever when it's closed,that would be for your A/C fast idle,it should have one wire hooke to it as well,
it could still be a spare,never know! Later,KillerDave