Hi All. I thought it had all gone quiet on this topic. Since my last posting, I have carried further research and in my opinion all 'ZA7S' plunger frames up to 1952 had the screw in adapters, but the 1953 model year (BA7S series) had the cheaper to manufacture plain 'top hat' type of adapter with a heavy washer on the inside to retain it in the frame, (this washer is not shown in the parts listings at all). I also believe that BSA carried over the part number to the later type as it features on both the early and later lists. I am currently looking into having a small batch of these manufactured exactly as original but in stainless steel. I will update progress shortly. Regards, Peter