As I recall, the first big resurgence of all things muzzleloading was in 1965, with the centennial of the Civil War.
Then of course, the Jeremiah Johnson movie with what's his name.
Along about that time, I started selling stuff at rendezvous and shoots.
By 1976, we were doing pretty good and then the Buy-Some-Tennial hit and things really went wild.
I would say that the centennial and the bicentennial were the big growth spurts, the movie was secondary.
Other movies like LOTM, the Patriot, etc. spurred growth, but the anniversaries were the things that really caused excitement.
Just my opinion, of course.
John