: Thank you for your very informative reply.
: I cannot find any similar event, so hope this case is an unusual
: decision. Otherwise, does this mean that a U.S. internal flight,
: which does not go through Customs, is prohibited too, or can it
: be safely packed in someway?
: I would still like to buy the item to put on the dash of a
: date-related classic rally car but cannot see a way to do so.
: Any advice greatly appreciated.

That's another good question and I don't have a real answer for you. Security for domestic flights in the US does not seem any less rigorous than for International flights. That said I don't know whether they use the same scanning equipment or not (or if it varies by airport for that matter). Could be a new protocol or your piece just got caught up at random. Also they might have different standards for cargo than for personal luggage.
Almost sounds like you need to research US Customs' requirements for this kind of material and see if there is a form or the like that will get you a dispensation to ship it internationally ('watch for repair" or the like). FedEx, UPS or DHL etc might be able to help, at least as far as pointing you in the right direction.
Sorry not to be of more assistance but this is also the first I've heard of an issue like this in about 13+ years of shipping & receiving vintage watches internationally. I almost wonder if the package was clearly marked as an old timepiece in the paperwork or if its nature was concealed and they bounced it back because they didn't have context?
Best of luck,
Tom