The two links to britishbikebits both state that new pattern parts are being made with a thread of 5/8" 20 tpi. That agrees with the stock my local supplier has. But the stock at my local supplier does not fit correctly on the gearbox mainshaft that my local supplier has in his shop, and they do not fit on my mainshaft. If it was only my mainshaft that had problems accepting the nut, I would be looking closely at the thread on my mainshaft, but to have the same problem on a different mainshaft, and I have spoken this afternoon to another A10 owner who had similar problems with the nut not fitting on his mainshaft, I suspect the new nuts are not being manufactured with the same thread as the originals.
To add confusion, the factory list of all BSA thread sizes quotes the 4 spring mainshaft nut, part number 42-3267 as: " Nut, CEI, 9/16 - 20, Mainshaft A10 4 spring mdls." That suggests a 9/16" thread size.
The 6 spring nut is quoted as: 66-3787, Nut,
,
, Mainshaft B31 B32 B33 B34. (The size and thread pitch are not given.)
If the Admiralty thread is the same tpi but cut a bit deeper than CEI would that make the fit of the nut on the shaft a bit looser than if it is CEI? That would solve the problem. At the moment the nut appears to have the correct tpi but is just very tight.