Kauffman's book "The Pennsylvania-Kentucky Rifle" lists some Kellers on page 274. Elias Keller at Yardleyville, Bucks Co, 1861. John Keller at Carlisle, tax lists 1823, 1832, 1842, 1844. John W. Keller at Carlisle, Cumberland County, 1861. I'll look through more of my books to see if I find others.
Whisker's book Arms Makers of Western Pennsylvania lists Joseph W. Keller (1821-1879). 1844 in McConnellsburg, Ayr Twp., Fulton Co. 1850 in Meadville, Crawford Co.
Frank M. Sellers "American Gunsmiths" lists 19 Kellers but no Jacob and none in the 1700s.