There's nothing at all puzzling there. Its a very nice New England rifle, probably made in Worcester County (because about 90% of them were). Probably less than half of the NE rifles are signed. The half stock is perfectly kosher, as is the horn nose cap. The target rear sight was almost certainly added. It was a flintlock, a late one, probably c.1816-1820 and the conversion is fairly early, probably in the 1825-28 period.
Attributing it to a maker is likely a fool's errand. With effectively all the rifle makers in worcester county, the rifles are remarkably similar, regardless of who made them. There are minor nuances people often take as "signatures" but this is all pure conjecture. I've had two rifles that were essentially identical, one signed and the other not signed. Were they from the same shop? Probably, but I wouldn't stake my life on it.