As a collector of percussion over under guns for many years, this is a new one for me ..... the closest gunsmith that I know of in Muncy, Lycoming County , Pa is J P GUYER (and later he added his son, Clinton Guyer). J P used a stamp on his work that is not at all similar to your barrel marking. Also, an 8 gauge barrel on a combination gun is EXCEEDINGLY unique; the largest I have in my collection are a couple 10 gauge. I suspect that your gun is a product of a part time gunsmith very late in the percussion period, maybe 1880's or 1890's. A couple photos MIGHT help, but I doubt that there is much to be found about this maker. Good luck in your search. If you do find more information, I would like to add it to my informal listing of percussion over under gun makers