Hi all. This has stumped me so I'm posting here to see if anyone can help.

I've got a BSA No 9 aperture sight fitted to my long Lee.
Fitted to it I've got single aperture eyepieces that screw in.
I recently bought a multi aperture eyepiece to use instead that is marked up 'Parker'. However.... This screws in about half a turn then locks up.

So, up until this point I had assumed all BSA (made by AG Parker / Parker Hale), Parker Hale and AJ Parker sights used a common sized thread. Physical evidence is now telling me otherwise, can anyone shed any light on this please?

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