First, the Elwell lock. I know Sellers listed Elwell in Seneca Co, OH, citing Hutslar. I'd love to see where Frank got that! I double checked his first book, published with Sumway, decades ago, and cannot find anything there except a notation that a Marker long rifle had an Elwell lock. Sellers perpetuates about every known error in "listing" books, adding plenty of his own. Horsehockey! This is perhaps most grievous error in Sellers & should be corrected [altho you can bet it won't be].
I put together the 5 vol set which the Assn of Ohio Long Rifle Collectors paid for, published & distributed. Nothing in it about Elwell as a lockmaker in Seneca Co.
As to the similarity twixt Leman's work + Deringer. Recall that Henry spent c.1831-34 at Tryon plant nearby learning mass production of arms. I'd give about even chances of the gun shown being a low grade arm sold to various dealers -- as opposed to "trade gun." Possible but I'd sure not buy [or sell] it with that as a promise!