There are 2 rifles depicted there. The top and bottom views are of the gun with the pointy-tailed lock and Pennsylvania-styled guard, if I am seeing it correctly. Would love to see the patchbox.
Its shown in Bairds book.
I suspect one of the rifles pictured in BBHC photos was always percussion.
The other rifle was almost surely a match rifle. In over the chunk matches the flint ignition may have been an advantage.
Below is complete suppostion but a couple of long time serious shooters I know subscribe to it as well.
The percussion system actually had teething problems. I suspect that until well sorted out and the caps were nicely uniform that the flintlock would out shoot them. Less variation on ignition. George points out that while the English shotgun went from flint to percussion almost overnight due to its great advantage in wing shooting. Apparently the conversion of rifles took longer and flint rifles were being made at a time when the flint shotgun was simply dead.
I think it was accuracy related.
This would be a good reason for the 15 pound Hawken "Kentucky" being made as flint. It could then have easily have been converted to percussion at a later date by simply cutting the barrel and installing a patent breech. New guns with flint ignition were common in America into the 1840s it seems.
The 15 pound rifle may have seen a lot of shooting and the vent got too large and the conversion was done. The possibilities are nearly endless.
From all this I vote for the Hawken with the converted lock being made as a flint and then converted, perhaps by Jake or Sam at some later date.
Dan