That cable you removed from the panel where the AC vent would be is a hand throttle. One year only, 1973, had the optional hand throttle mounted in that location. That year only you could get AC or the hand throttle, but not both. 1974 and on the hand throttle cable was mounted low on the dash, right above the foot throttle.
Cable operation: Steel spring loaded ball inside set on a coarse threaded cable end. To operate it, pull hard enough so the spring ball jumps onto the threaded part of the dash cable end. Then you can spin the the knob to fine tune the RPM's wanted. Like you stated, when the knob is all the way in, it is not yet on the threaded part of the cable end, so it just spins.
It's gone now. Thank you.
If you don't want it, I will buy it.
Sorry I didn't see this post sooner, your description was spot on from the beginning.