Your gun looks remarkably similar to my Thos. Forsyth hammer gun, with the biggest notable difference being the lever arrangement (side vs. under lever). In fact, aside from the engraving, the locks appear to be identical.

http://www.doublegunshop.com/forums/ubbt...8679#Post398679

I have had no luck in discovering the provenance of my gun in the two years since I wrote the original post. I hope you have better results.