Mark,
If you are refering to the knurled knob that is visible to the pilot, of which there are 4 on each panel, it is 4 F 109. I got 8 from Bob Powell five years ago. I don't know if his successor still has some in stock. They are held into the panel by 3 left hand reatainers C1-F101 and one right hand retainer C1-F102.
regards
Nigel