Do you have any idea about the Proof Marks? Or do you know anyone that might have some idea what they mean?The Mortimer name has a fairly long history in British gunmaking/retailing and even in the US at later dates. As with all British firearms the actual maker is suspect. Mortimer could have made the entire gun, just the locks or, none of it and had it built in Birmingham "for the trade" for his shop.
If you could find someone with the book by Boothroyd's I feel certain there would be quite a bit of information in it. He and his wife covered pretty much the entire British gun trade...and I don't believe they missed any. Thing is, the book is scarce and expensive.
Your gun looks to be in remarkable condition. The time period is the last quarter of the 19th century. I'm not much of a shotgunner, being more of a rifleman so my guess as to value is suspect. However, judging from other guns of like kind of the same era and condition $1250.00 would be the minimum value, in my mind and, I suspect more.