4511 1/4 inch 6-1 drive without shaft for a dial knob.

This is the drive that we use in our 4489/C unit, its basically a standard drive with the shaft cut flat and a d shape , it allows you to add a dial knob or something else. 

We sell a suitable knob as well, I normally use a single screw and a washer to fix it, that's a separate item. its 70mm wide 27mm tall