Early Model 700s like this 1962 .243 carbine have 5 digit serial number, this one is # 671XX.
The rifles were button barrelled, very accurate, and yes, some were manufactured by Hart.
Value? It's in the eyes of the beholder.
Me? I think that in the 60s Remington was producing a fine rifle at a competitive price that upset the Winchester accountants and that Rem. continued to produce quality firearms into the 90s when their own bean counters ruined the company.