Levis began in 1873

Levi Strauss began his company in San Francisco in 1853, and patented his "Levis" jeans in 1873. "The Searchers" is probably set in 1876, considering the references to Custer and the fact it's based loosely on the Cynthia Parker story, so they could be wearing Levi's brand jeans.

Levis have been around longer than that

The Searchers takes place after the Civil War. Levi Strauss started his company during the California Gold Rush in the 1850's, so the characters would probably be wearing Levis. On the other hand, I believe the red tab did not exist at the time.

martha hyer???searchers????

martha hyer was not in the searchers. Vera miles was the young blonde in the levis

Clarification

According to about.com, the red tab was added in 1936 to help ID the jeans from a distance. Good nitpick!

Further Clarification

I don't think the movie is quite to 1876 yet. It begins in 1868 according to the opening and Vera Miles states on Martin Pauley's visit that he has been gone for two years. That would make it 1870. Furthermore, I could think of no Custer references in that film. Of course, if going by the appearance of the Colt SAA then it would have to be after 1973, but just about all westerns make this mistake.