Yup... I have one. It's just that we don't think Navy Arms was making them in the 1950's or early sixties, as the shopkeeper told the OP. In addition, my musket has "Navy Arms" prominently imprinted on the lock plate. The OP indicated he could not find any markings to reveal the maker or importer, so it may not be a Navy Arms gun. Hard to say without taking a look.
I just bought mine a few months ago. It is an older Navy Arms M1863 rifle musket, made in Italy. I've had it completely apart, but failed to write down the date code and promptly forgot it. As for quality, mine is not the best. Respectfully, it has been my privilege to have worked on a few originals, and I'm here to tell you, the 150+ year old guns were better built in every way. I do believe the Italian manufacturers have upgraded their products since mine was made; the Euroarms Richmond shown in my post above is a much nicer gun. The inside of my Navy Arms Springfield lock shows
no attempt at polishing whatsoever. They used undersized common flathead wood screws for attaching all of the mountings (butt plate, trigger guard, etc.) while the originals and better reproductions used very robust, oval-head screws with a special thread. Barrel bands on the Navy Arms gun appear to have been formed out of heavy sheet metal, while I believe the original bands were probably forged. I could go on, but my early Navy Arms Italian-made 1863 shows a lot of manufacturing shortcuts. I haven't shot it yet, as I found the bore was much worse than described. Some of this gun's problems are from age and neglect, and I think it can be made into a decent recreational shooter with some work, but it was never a high-quality gun, even when brand new. However, I just got it as a "fun gun," so I'll deal with the issues.
As noted, I believe the Italian manufacturers have likely responded to competition and customer feedback, and upgraded their products in recent years. I think the Miroku (Japanese made) Springfields were likely better than the Italian guns, when the Mirokus were being made, and the Italians probably figured they had better tighten up if they were going to compete in the marketplace. I believe Navy Arms imported at least some of the Miroku guns, and they probably were pretty nice quality. So, I'm not disagreeing with
@Trot , but just qualifying my response. His comments are appreciated.
In any event, this may be a case where older is not necessarily better. It is best when a prospective buyer can actually handle and examine a used gun, as the OP is doing. He can decide for himself whether it meets his quality criteria.
Best regards,
Notchy Bob