Durs Egg went out of business about 1820,it's believed he went crazy from mercury poisoning,your gun is much later than that.

Some Birmingham gunmakers made lower quality guns and engraved them with fraudulent famous names,they were sold as used guns to get around the law. Mortimer was another name they used,I've handled a couple that I'm sure were never seen by one of the Mortimer's.Sorry about the bad news.