Its certainly a New England gun. The W Ketland lock has to date between about 1801 and 1831... which is a pretty large window. It looks to be converted from flint, but it is reasonable to assume many unused flint locks were converted at the time they were used, if after about 1825 they could be either flint or percussion. That said, I'd' date the gun around 1816-1820 though I freely admit I could be off ten years in either direction. As to the bore, they aren't commonly that large, but it isn't unique. Almost all NE flint rifles are around .54 caliber and I have a signed Maynard half-stock with a T. Ketland lock that is .69 caliber (with rifling still clearly showing).
Virtually none of the features of NE rifles will help you identify a maker... (there are some rare exceptions but even these are problematical.) The fact is, nearly all were made in Worcester county, likely all the makers knew each other and were familiar with each other's work and probably all of them bought their fittings from the same suppliers. There is no reason to assume they made their own patch boxes & trigger guards though some certainly may have. If they did, they rarely differed from the prevailing styles.