I have a trip to the Mercer Museum in Doylestown planned for next week. Actually, for the very same reason.
My interests are Upper Susquehanna rifles, primary Clinton County. As you mentioned the home and shop of Seth Iredell Nelson Sr & Seth Iredell Nelson Jr was on Round Island, Sinnemahoning Creek in Clinton County, PA.
I hope to come back with pictures of the rifles if they are still there and available. I'll let you know and share.
Thanks,
O.R.