Davy Crockett and Daniel Boone did not have SN’s on their rifles (or pistols).
Do not be so sure about that. Manufacturers started putting serial numbers on their products before the US Civil War. I believe it was for their own purposes, most likely inventory control.
I would also consider it very likely that even if some of the various guns both men used over their lives had serial numbers many had no more than a manufacture's mark (brand symbol).