The DPO is a term you need to know and remember (Dumb Previous Owner) or maybe not so Dumb as this one got a 60 year old bike running again with the original part not available. I take it the proper nut is for a larger diameter, so the DPO tuned the worn thread off, cut a new smaller diameter thread that matched a nut he had lying round. So your choice is new gearbox mainshaft so the proper nut will fit or reuse the odd nut on the modified gearbox mainshaft.