I personally feel and have always held the belief that folks who started out with flintlocks - simply stayed with the flintlock ignition as it is what they knew and were comfortable with. Also, they didn't have to worry about an extra "percussion cap" expense as nature was their ignition supplier because they knew how to make a flint for their rifle's lock... (Early percussion caps didn't have a great reputation of always setting off the charge I read somewhere at some time over the last 40+ years)
This could be a rifle of the "mind set" of a person having their (fullstock) rifle rebuilt (for some reason) and some of the parts being re'purposed to keep the price down. The article stated there was no sign of this rifle ever being a caplock...
My own halfstock rifle I converted from cap to flint is based on the paragraph above.
The brass tacks are interesting - though I'm not a tack expert so I don't know it they tell a story,,, and when they were added would be anyone's guess?
We'll no doubt never know for sure the history of this rifle, but its caliber is certainly in the ball park of a originally known caliber from the western fur trade era of 1825 - 1840 (and beyond).