The front sprocket nut?
There should be no wear on the sprocket nut. Perhaps it could get damaged due to over tightening perhaps, or perhaps being removed if it's bonded a little, but really it should be fine. The only wear should fitting/removing and erosion.
The original 600 was well known for the front sprocket coming loose. Mine did, which was before we all realised it was happening right left and centre. My front sprocket and washer were mince and the output shaft shagged as well. I welded a new sprocket on and traded up to the thou.
A couple of things. Is this still an issue on many models? Or just something to be aware of at service time, particularly when the chain has been on for a high mileage.
Also of the nut is worn, I'd be concerned about the o/p shaft, did your mechanic make any comment on this Grahamm?
That said if it is damaged, but cleans up and takes the full torque setting, it should hold fine.
Anyway when I replaced the chain and sprocket set on my thou, I put some stud lock on the threads. Don't fancy riding about with just the chain tension holding the sprocket in place again!
Oh and don't forget there are two different DID X ring specs (I think it's just two) VX (value X ring) and ZVM (super strong long lasting). They come in black, silver and gold, though everybody seems to sell the gold!