Wow is that some dated information, Herb! But it is the same vintage as Baird's writings, in fact Baird has some of the same wrong dates in his book.
Daniel Hartzler (with some contribution from George Shumway) conducted some research that totally changed the date Jacob arrived in St. Louis and his relationship with Harper's Ferry. It was published in Hartzler's book,
Arms Makers of Maryland, in 1977 and summarized in Hanson's
The Hawken Rifle (1979).
Jacob didn't arrive in St. Louis until 1818. The Harper's Ferry connection with Jacob is even stronger than the whimsical speculation that Barsotti and Russell wrote about. Jacob actually worked at Harper's Ferry from 1808 to early 1818.
The rest of Carl P. Russell's discussion of Hawken flintlocks is pure speculation, too, because none from the early years in St. Louis are known, especially half stock flintlock Hawken rifles.
Some of the info about other gunsmiths (in his footnote 4) making half stock flintlock rifles isn't far off. I don't know about the B. D. Gill rifle(s) he speaks of but have seen pictures of such from some Philadelphia gunsmiths.
Here is one made by Jacob Kuntz. It even has a paneled beaver tail cheekpiece like some early J&S Hawken rifles. Kuntz was probably influenced by English sporting rifles of the time (Philadelphia gun makers adopted a lot of British traits such as the pineapple finial), but he "Americanized" it with a crescent butt plate, Pennsylvania trigger guard, and engraved brass patch box.
Speaking of HF 1803s, I saw this rifle at Jim Gordon's museum, and it blew my socks off. Gordon has no history on the rifle, so we don't know who made it, but it really stimulates my imagination. It was made with mostly HF 1803 parts, including the lock dated 1806, but has a full octagon barrel and a period re-stock with a beaver tail cheekpiece, even.
People have long noted and argued over whether the Southern Mountain Rifle or the Harper's Ferry 1803 or the British Sporting Rifle influenced the Hawken brothers. I think it was all three.
