Percussion guns made by James P. Guyer occasionally show up on the market. The few I've seen are rather plain, solidly built workingman's guns. He made both single barrel and also over/under double barrel types. My O/U is .45 rifle over a .54 rifled barrel. My buddy down in Pennsylvania also has an O/U by Guyer that I remember as a rifle barrel over a rifle barrel.
I believe James Guyer was born in Muncy, Lycoming County, Pa in 1832. In 1850 Guyer was an 18 year old apprentice to William Miller in Danville Borough, Montour County, Pa. By 1860 he had set up his own gun shop back in Lycoming County. His son, Clinton, patented a breech-loading rifle design.