This was Jeff Connors', Chuck Connors' real son, third and final appearance on the Rifleman.
This was Glen Ryle's unique appearance in television or film.
Lucas points out that Mark is 13 years old. At the time this episode aired, Johnny Crawford was actually 15.
Though filmed for the time period of the 1880's, Lucas is wearing Wrangler jeans that weren't invented until 1947.