I could swear I read someplace reputable that the 1883s were built in England ... that Colt's son wanted to market a really fine double, couldn't justify tooling up for it in Hartford and so decided to get the ball rolling with a nice Birmingham-made boxlock. I assumed that the same would be true of the 1878, but must admit, I haven't been able to find the source for any of this in my own library. Maybe it's just a fairy tale. To make matters worse, I once owned an '83 but can't remember anything about proof marks. What some here have suggested -- that it would have Brit proof marks on the water table -- certainly seems logical.
Ceruzzi: When we talk about a qualified gunsmith, most of us would agree there's only a relative handful who know enough about old doubles to matter. Like most of us, you'll be shipping the gun, rather than driving it over. I'm frankly pleased to hear you say you should give this gun more thought. Not that it might not be a great gun, but picking a 10-bore hammergun with lots of drop as your first pheasant gun is not ideal ... a bit like learning to swim with an anvil on your back. Tons of really solid expertise here and some great guys willing to share what they know. The money you're willing to spend can get you into a really nice gun that will be collectible and very huntable, so take your time and keep asking questions.
TT