Gaintwist,
Why don't you call Hart's? They can also tell you when it was made. Please post what you find out.
I have the same action about 100 higher serial number, and I've wonder the same thing. Not enough to call, though. My rifle is pure stock Hart 1970's vintage, wood stock, un-bedded, Hart trigger, 222Mag, with a Lyman 20X (boosted to 30x by Wally Siebert) on it.
Regards, Ron