I wasn't aware that there are a lot of reproductions of this specifc seat. It is not a particularly rare one. Not to say there aren't though. Not an over valuable seat - maybe £50 at auction.
Had mine for years - definitely original.
Generally easy to recognise repros - usually much smoother and not weathered. I'm afraid this reproducing is plaguing the hobby- but I don't believe that it is quite as widespread as it is made out to be. For one thing over here foundrys (thanks to health and safety and the b............ environmentalists) are few and far between and expensive. Quite a deterrent. If they reproduce them with aluminium which is more easily done "at home" then they are easily recognised.
The two seats that were done for recent Massey Expos were aluminium if I recall correctly