Looks like sterling to me, perhaps gold filled, plated. washed, etc... Cool item, looks to be a pendant locket to be worn around the neck on a chain and store a picture or other item in it.
The marks look English but not sure. The first mark (after flipping/rotating image) looks like the English Standard mark of a Lion, The Second looks like the City Mark of a Leopard, can't tell if it's crowned or not. The Third looks like the Bust of a Duty Mark, can't tell which one, the Hallmarks would have to be cleaned. It also looks like you have the Makers Mark on the inside Flap of the Envelope, clean that up too (I would, otherwise take a jewelers loop and look at the marks, it's your call really. Could lose the Gold wash though, if that's what it is).
Try looking your mark up here, if this link doesn't find it then search around on that site.
http://www.925-1000.com/british_marks.html
