Both my '46 trucks have the factory 'T' knob.. only used it a few times on real cold days when I wanted to run higher than normal idle RPM's to warm up the engine.
As Red points out, it would be useful in the larger trucks with PTO's for running the pump.
A '66 is 20 years into the future, so technology could have changed things.. but in '46 the 'T' knob is connected to a cable the same size as you'd find on your typical lawn mower throttle cable, and connects to the throttle on the carb.
I can't imagine it would be a real accurate, reliable, or safe cruse control, tho.
