David - your best approach may be to get familiar with some of the more popular collector firearm auctions and search those. and a google search for specifics of what you are looking for can locate one in an auction also. There are not many around, so it may take some patience. If my memory serves, they may be marked “IK”. Not sure I ever saw one of them from before the 1808 contracts though you are correct they manufactured them back to the rev war. I believe they worked in montgomery county pa or eastern berks county near douglassville. Watch out for muskets that have been restocked or may have a replacement lock, if you are looking for a mostly original piece. Best of luck!