Roger, I would guess for the most part just how much the guy was asking for the gun.
If it's only a couple hundred bucks I'd guess it doesn't really matter if it was English or Belgian made, especially with the engraving quality you mention. From your description of the gun, you knew you weren't looking at a first class Manton!
I guess what I'm trying to say, is that if it isn't in good enough condition to make it worth more money dependant on the maker or origin, what difference does it make.
Then again, maybe the seller thought you might break it taking it apart, and then of course, only offer him half as much for the gun, since it's busted and all!!
John