If memory serves me correct, the serrated handle drive just slips on and off under the dial/ nut assembly. It has a shoulder that fit into the dial
It may be rusted or just gummy from years of use. I remember that if you unscrew the dial nut it will force the handle driver to start to move. There is just a key way that the driver drives off of to the shaft to move the knee.
I don't remember a set screw on the driver.