I don't know about the diameter issues, although the knobs on older cameras like an Automat and on the 'Cord V look similar.
I do know that the focusing scales on the 'Flex (left side) and 'Cord (right side) go in different directions. On either side, to focus closer the knob turns 'over the top' from back to front. Counterclockwise on left side, clockwise on right side.
I have seen extension knobs with scales in both directions. The first time I was about to buy one, something seemed weird and i kept wondering what was off until this different direction of the scale became obvious. In that case, I was looking for a left-side knob but the scale was for the right/'Cord side.
I have seen them listed on Ebay as for 'Rolleiflex and Rolleicord.' I think the 'Cord Va was the only one with a small left side knob?
If the scale numbers aren't important, or having a larger knob is more important than the scale, you'll need someone else to tell if the extension knob will fit on your knob.