Thanks for the explanation. Hopefully the lens will turn up tomorrow then I can take it apart and play with it for a day or two (and stare at it a lot!) before deciding what to do.

Was a square platform mounting area the only reason for reversing the helical?

If I kept the helical the correct way round and machined a flat surface to mount a lens board/disc to, would the existing slide still work or would it be in the way of the rear elements?

I know I could wait and answer these questions myself but I'm impatient!