I apologize for reopening this post, but it's an interesting topic!
I think one thing that has not been touched on as yet, and that is that the Mountain Man era, although credited with ending in 1840 after the last Rendezvous,,, well it really didn't... The fur trade continued, it just wasn't as lucrative as it once was, and the annual gathering (Rendezvous) where no longer taking place. However, the trading post were still active and some new ones even sprung up.
Now I do agree completely that there were probably darn few Hawken half-stock rifles during and up to 1840 in the hands of mountain men. And, it only takes one living "famous" mountain man to buy a new half-stock Hawken Rifle (towrads the end of the given 1840 date, or soon after that) to bring forth the notion that all mountain men carried one. We do know Kit Carson had one and remember he was in the public eye which had much to do with him guiding Fremont.
Bridger opened a trading post if I recall, and those stopping to resupply along the Oregon trail, had they seen Bridger toting around a Hawken half-stock rifle, to those unknowing souls set on the trail for Oregon wouldn't know the difference IMHO and would just assume he always had one, even back in his trapping days. Who knows, Bridger himself may have spun some tails to those folks stopping at his trading post about the wonderment of his Hawken Rifle and the fixes it got him out of? Pure conjecture of course, but possible.
There are plenty of ways to spin a myth, and sometimes those myths become the twisted facts of history. Eventually things start to get sorted out over time, and it starts opening the eyes and ears of those interested in such things... Like many of us here, are.