This shot is from about five minutes into the episode "Border Corsairs." Kit, played by Bill Williams, is on the left and his sidekick El Toro, played by Don Diamond, is on the right. This shot is in fact flipped.
Around the 10-minute mark of the same episode, this shot appears. The riders are now on opposite sides of each other. But if you look closely, you'll see that it's not just the riders who have switched sides — everything is reversed from the earlier shot. In the second shot, everything is correctly oriented. One of the better giveaways is that the curved white marking along the nose of Kit Carson's horse curves to the left in the first shot and to the right in the second.
The main feature in the background is Oat Mountain, the light-colored series of hills along the top of the shot. Another way to tell the shots are reversed is by looking at the dark triangle shape in Oat Mountain, just above the riders.
Here's another example of flipping the shot, from "The Roy Rogers Show."