Lemuel G. Hill was born in England in 1812 or 1813, and was the son of Samuel W. Hill, also a gunsmith. The family moved to Johnstown in 1821. He first worked with his father, who is believed to have died during the late 1840's. Lemuel worked at least as late as 1878 and died April 30, 1895.
There is further information in The New York State Firearms Trade, Vol. 2 by H. J. Swinney/Tom Rowe. There is an S. W. Hill (flintlock) rifle pictured in the book, but none by Lemuel, although two are described.