For what it's worth here is part of an article from the Feb 1955 edition of "Guns" magazine:
(just a few years before the "hype" of Jeremiah Johnson
Technically, the first Hawken caplock rifles seem to
have been developed by adopting features of three flintlock
rifles of the period just preceding: the time-tested Ken-
tucky rifle, the Harpers Ferry rifle first made in 1800 and
improved in 1814 and 1817, and the heavy English
sporting rifles produced by London gunsmiths since about
1800.
Very few English rifles had reached the American
frontier in 1822; the Harpers Ferry was fairly numerous;
the Kentucky most numerous of all. In 1822 the long-
barreled Kentucky was considered the ultimate in perfec-
tion by gunsmiths and shooters alike.
Daniel Boone and
Simon Kenton had carried the Kentucky in the Indian-
haunted woodlands of the "Dark and Bloody Ground,"
and the famed weapon had served America with brilliant
distinction throughout the Revolution and the War of 1812.
The superiority of the Kentucky was an unassailable tra-
dition in 1822.
Yet, west of the Mississippi, the renowned Long Rifle
of the early pioneers proved clearly inadequate for grizzlies
and buffalo. Hunters, returning to St. Louis from the
mountains, demanded of gunsmiths a rifle capable of
dropping a fat buffalo cow or of stopping a charging
grizzly.
Jake Hawken, alone among St. Louis gunmakers,
produced the needed weapon at the right moment and his
market was ready and waiting. Mountain men were avid
to buy the rifles as fast as he could turn them out and
Hawken swiftly became famous.
The article also notes Jake's (dislike) of the flintlock system and suggests he believed caplocks would be superior is wet/windy weather (quotes a supposed conversation between Jake Hawken and Bill Williams (famed trapper/guide) who "according to this article" bought Jake's "first" caplock rifle).
side note - IF that conversation took place AND Bill got his first caplock, either this was Jake's first rifle built in St. Louis OR he must have built flintlocks. If it was his first rifle would a "famed trapper, scout etc" be the "guinea pig" to a rifle maker with no "proven models"??
More fodder for the ever raging Hawken debate..
(I have the magazine in PDF format if anyone would like a copy)