Often you see Lucas riding a horse using a Quick Bit. These bits didn't exist until the 1950's. Became a well known bit outside of California by the 1960's.
My guess, the Rifleman takes place in the late 1800's?
It takes place in the early to mid 1880s. Their clothes were sewn with machines, and there were probably some zippers in there, too. I've spotted machine-made whiskey bottles in the bar, which didn't exist until at least 1903. McCain's rifle is a model 1892. Everybody uses modern smokeless gunpowder, which didn't exist yet. "The Rifleman" utterly fails as a documentary.
But still, it's fun to spot these mistakes. It adds to the entertainment.