The Fictional Date

If I remember correctly, someone thought or said in the original novel Mackenna's Gold , 1963, by Heck Allen, that Nana was dead. Nana is mentioned in many versions of the legend of the Loast Adams Gold, so there would be reason for a character to think about whether Nana, who allegedly knew the location of the lost valley of gold, was still alive.

https://en.wikipedia.org/wiki/Nana_(chief)

In real history Nana died in 1896. So that would seem to put the original novel in or after 1896. Even if Heck Allen didn't know when Nana died, he should have known that Nana made his final surrender in 1886, so if a character thought tha tNana was dead, Heck Allen could hardly have imagined that the events in the story happened before 1886.

but I saw Mackenna's Gold (1969) a a few days ago. the Narrator mentions the legend:

Everybody knew about that legend,
and a lot of people believed it.

"Canon Del Oro."

"The Lost Adams."

And then for a while there
back in 1874, they called it...

"Mackenna's Gold."

The movie transcript is here: https://subslikescript.com/movie/Mackennas_Gold-64615

So Mackenna's Gold (1969) seems to have a fictional date of 1874.