There are 57 William Williams listed, but only 1 was for the 26 USI and the notations show that he was alive in 1815 so he can't be yours. Only 2 are listed as being born in PA and neither fit yours according to the notations in their records. Birthplaces listed for the others range from New England to the southern states.Were there other regiments from other states that were included in the formation of the 19th Regiment? Some of the men enlisted in states other than their birth state-for example a man born in NY enlisted in VT.I know that families were migrating west from both New England and the south so that can make it tough. There were also at least 2 William Williams 1 born in England and the other Wales, so you can see the difficulty here.There were also several Willey Williams, but they didn't fit with yours either. If you have any other ideas or information that might provide a clue, I'm willing to help if I can.