Any way of telling if the chain and sprockets on my 77 rd400 are original ?
You could try carbon dating. I joke.
Honest question, why does it matter? Are you just trying to judge originality of the bike?
Condition should be commensurate with mileage on bike. I've seen originals (chain and sprockets) with 10k on them that were still serviceable.
I guess if the chain is a non-Oring 530, with the stock sprocket sizes, it COULD be the original..
But I agree with sav0r, why worry about that?
In any case, I wouldn't even consider using a 530 chain on my RD today!
A 520 X-ring chain, and an alloy rear sprocket saves quite a bit of weight, is plenty strong
Here's some pics of the oem 530 chain sprockets
(https://i.ibb.co/1sR1dSR/sprocket.jpg) (https://ibb.co/7grPR9r)
(https://i.ibb.co/Z212W86/sprocket-a.jpg) (https://ibb.co/bbRbzBg)