The only "standard' thread on a SMLE is the stock bolt inside the butt ( 7/16' Whitworth), (ish). . Everything else is an "Enfield Special", regardless of whether made in Britainicon, Australiaicon or India.

However, there are still a few bits and pieces floating about.

The little bit you are missing is called the NUT, Vocab Number BB0700 and Drawing number SAID 2030.

The thread is one of those "specials", i.e.: 0.144dia x 37tpi.

Try Alan Hibbs in Kingaroy for starters.