ACullen
Well-known
Some 50s lenses used 34.5 diameter filters. If 34 is a little loose, I’d suspect this was it.
Thanks for that info. I didn't know that size existed. I'll see if SRB UK can make a step-up ring.
The lens for anyone interested seems pretty decent. Decent central sharpness wide open. Very sharp centrally by f4 . Edges sharp by f5.6. Great contrast too. Handling is good, smooth focus , click stop apertures. So other than a PITA filter size , a very nice small lens. No real idea why it might actually be useful. Time will tell if this GAS leak was worth it!