There is no requirement in the United States on the Federal level for a serial number to be affixed to an "antique firearm" and repros of caplock or flintlock firearms, even if they are made 30 minutes after I post this reply, are all considered "antique". Now as to state level, depending on what you are discussing, the state may want a serial number on the "antique firearm"...I'm thinking New Jersey may want them on cap-n-ball revolvers, and so might New York City, but don't know for sure..., and it might only apply after an injury involving said piece had happened. Meaning possession of such a piece might not be in an of itself a violation.
Serial numbers DO matter if one is buying new, and fills out that "stupid little card" that then registers the item for warranty purposes. It also matters for dating of when the item was made for some manufacturers.
Wile some muskets benefit from extra work, that may not be true for all guns/rifles used by reenactors. For example my Pedersoli Bess was "defarbed" before I bought it, and due to the seller's pictures and the fact that the previous owner allowed some rust to build up..., I got it for the cost of less expensive, well used, unaltered Pedersoli Bess. So the butt plate is retro fitted to be an older style, and the side plate is rounded, and the lock had the engraved date changed to the 1750's. All the barrel markings were removed save for the serial number. So in my case YES the mods would and did add value.
But..., say for instance, normally inexpensive cap-n-ball revolvers..., for discussion lets use Pietta "Confederate Navy" revolvers..., which never really existed, as they look like Colt Navy revolvers, BUT they are in .44 and have brass frames. They were literally the least expensive cap-n-ball revolvers on the market, AND they were also on sale when I bought them. They were part of a magazine article on the lowest possible price for entering Cowboy Action Shooting. In fact the article explored the possibility of the shooter actually buying extra cylinders to reduce loading time at the bench at a match.
So..., If I was to sell the set, for it consists of parts, an "Avenging Angel", and two full sized revolvers, plus nine spare cylinders.
So I could sell them to another CAS shooter..., I could sell them to an ACW reenactor who didn't care that they are fantasy revolvers, as he or she would be very "well heeled" after the purchase..., or I could sell them to a person who likes cap-n-ball revolvers..., or I could split up the set. The only things done to them was the removal of the blue and then a mild browning to make them look well used.... and the one revolver which had the barrel shortened and crowned. Taking off the Pietta factory markings would not make these fantasy revolvers any more valuable, and the antiquing was done for another article. The ONLY way these have ever "increased" in value is due to 25 years of inflation of gun prices, which may, today, allow me to recoup what I paid for them but even that sum does not have the same buying power that it did in 1995, eh?
LD