thicker sprocket nut worth a check

[Adam2201]

It might be original then - 30k seems like a lot though!  Will get a pic of the other side when I get back home.  The chain has 5DVN Japan printed on it and definitely has a rivet link though, I think the originals were endless?I remember it being fairly difficult to undo the 9mm nut - I had to put a pipe on my breaker bar and lean on it, and when it let go I got the shotgun effect you mentioned (not as bad as the nut on the clutch adjuster though!).
 
I think the bike had a Scottoiler at some stage, as when I got it the vacuum cap on #1 carb was missing, so it might explain the longevity if it is original.  I use spray lube once a week or so, but I've not been fastidious about cleaning it so it's done well really.