Finding the handwritten note at the site of the rendezvous mentioning white women actually happened in history and they should have said the women were missionaries traveling with their husbands to the Oregon Territory. They were the first white women to go that far west and cross the continental divide. Therefore, finding the handwritten note can be considered a milestone in the history of the expansion of the American West.
Though it's never explicitly stated in the film, the story takes place during 1838. This is based on the fact that the beaver market was declining and the rendezvous was held on the Popoagie River. The "Era of the Mountain Man" ended two years later.
Many of the extras in the rendezvous scenes are reenactors known as "Buckskinners." These folks had a grand old time making this film. Most of the costumes seen in the rendezvous scenes, including weapons, and many of the horses, were the personal property of the actors wearing them.
One of the scenes in the film is based on an actual historic event, the famous run for life by John Colter. John Colter, who, prior to this event, had served with the Lewis and Clark expedition, is considered by many to be the first authentic mountain man. John Colter was captured by Blackfeet, stripped naked, and forced to race for his life. In real life, just as in the movie, John Colter escaped by killing an Indian who was closest to him with a spear, and hiding inside of a beaver lodge in the river.
During a 2019 interview in the podcast "Inside of You with Michael Rosenbaum," John Glover said that it was a complete surprise to him when Charlton Heston chose to film one scene completely naked. "I didn't know he was going to take all his clothes off for the shot. Well, we'd not rehearsed it like that. . . . I ride up on a horse and he was standing in front of a campfire, and as I rode up I could see that he'd taken ALL his clothes off. So as I walked up to him, I kept thinking, 'Now, where do I look!'"