Thank You Brian for your thoughts...
arghhh...Yes looked up Nikkor RF 21 prices / crazy prices
Igor cameras has this which is possibly an F version one can use per your suggestion , F to S adapter ??
Nikor-O 2.1cm/4 #223919, retrofocus, Nikon NKT 2.1cm Finder, UV filter, caps, Mint $645.
Starting to feel anxious thinking about selling equipment, so maybe a CL or Hexar is the way to go
Voigtlander made F-S adapters a while ago, all sold out. If you can find one- yes, you can use it.
BUT- It will not be RF coupled, which for a 21mm is unnecessary.
The F-Mount 21 is usually found with the external viewfinder that mounts over the rewind Knob of the F and F2.
You "could" pick up a Nikon F or Nikkormat and just dedicate it to the 21 in F-Mount. Metal Shutters.
OR- pick up the Voigtlander 21 in S-Mount.