Dan, I think your correct about Newton barrel markings and the rifling. From the close-up pic of the muzzle it appears to be the "segmental" or round type Newton used on his model 1916. One sure way to find out is to see if the extractor cut continues to the left side of receiver. Newton made his barrels that way for gas escape to the left side.
The grip cap also looks to be from a model 1916 Newton. Check to see if there are serial numbers on it.
I don't see anything Jas. V Howe either. Notice how shallow the receiver was inletted at the breech. Didn't even need wood removed at the loading port.
