The T on the knob is for throttle. Originally they had a stud with a hole in it at the carb linkage. The cable had an end with a set screw in it that was to slip when you used the pedal. The idea didn't work very well and usually the throttle cable just dangled. We had use for them because we pumped off gas with a pto, but because they were always broken anyway, we stuck a coin between the stop at the carb and it served the same purpose unless you were penniless. There is a picture of the setup in the GMC maintenance manual 47-54. I can scan the illustration and email it to you.