Clutch retaining nut - late type gearbox

As is often the case when you start one job 3 others appear, my intention was to fix the gearbox oil leaks, while I was in the gearbox I thought I’d also do the sleeve gear as the clutch was a little sloppy! That’s when I discovered the damaged selector forks, while was reaming the the sleeve gear I noticed that the mainshaft was a bit scored, but there’s one on the club shop, so I bought that, I was putting the box together over the weekend, (with nice new oil seals in the cover) and came to fit the clutch, low and behold the retaining nut won’t fit on the end of the new shaft! Close inspection of the old shaft shows damaged threads. 

As far as I can tell the nut should be 9/16 20tpi cycle with a reduced hex (the same thread as the Navigator front wheel nut - I know I’ve checked!) My guess is the previous owner probably forced a 9/16 BSF on there. 

So the challenge is to find another one, I have found full size ones but doubt I’ll get a socket on it, I’ve ordered a cycle tap so I can drill out another nut a retap it ..... fingers crossed!  I should add that the club is out of stock, Norvil don’t list it and I can’t find one anywhere else at the moment.