Sorry, that gun is not only original, but all matching (in the visible numbers). As for being a repro, all I can say is that many folks today have never seen an original 1851 or know what to look for. The grip color, BTW, is either original or very close to the original. Colt grips were originally varnished and they used a reddish stain (as did the Henry/Winchester at about the same time).
Any site or book on British proof marks should show the London Gunmakers Company GP proof and view marks. Note I said the gun had London proof marks, meaning it had been sold in or imported into England. I specifically said that it was NOT MADE in London, being much too late, so it would not have the London address marking. Colt exported Hartford-made guns to England for many years, right up to the British handgun ban.
All the numbers are in the right places and more importantly in the correct font. The characteristics match the Fourth Model perfectly. (Check Swayze for the Model distinctions among those guns.)
There are clear signs that the gun was polished before bluing, probably to remove light rust, but it was not heavily buffed.
The good news for the OP is that he has an antique, original, gun in pretty good condition.
As to value, that always depends on the circumstances as well as condition. Considering the Navies that I see at shows today for $2500 (asking price) I don't think $3000 is too far out of line, but as always, value is what someone will pay for it.
Jim