According to the style of the hood in the pictures, you have a pre - 350,000 serial number. It would have been built between 1976 and 1979. As stated by others, the lever by your left foot mechanically engages the rear pto shaft to the hydraulic clutch. The yellow knob between the shift levers engages the hydraulic clutch when lifted up. Most people tend to leave the lever by your foot engaged all the time and start and stop the pto using the yellow knob. One advantage to the lever is when you disengage it, you can turn the shaft freely for aligning the splines when connecting an implement. If the shaft will not stop when the lever by your foot is engaged and the yellow knob is down, you likely have a bad pto brake or a broken oil line inside the transmission that engages the brake. BTW, nice looking tractor! you should get good service out of it.