I purchased a used 210mm Symmar S in Prontor#1 mounted on a Horseman lensboard, hence I need change it to my Toyo 45AX board. The retaining ring appears dead stuck.
I have tried to soak its thread with some white-spirit, then some paint-dissolver (containing aceton), then lubricated with a drop of machine oil, left it for few hours, then used a Linos wrench tool to unscrew. I applied all my force with two hands (while other person was holding the lensboard with the lens) -- no way.
Anybody can suggest a trick?