I don’t doubt that that is true — but I believe the series was supposed to take place in the 1880’s. Television has a long history being a little casual about facts and timelines.
I found a web site dedicated to the rifle used in the show that stated that the show takes place twenty years earlier than the rifle used was actually introduced. Would have made it 1870s. OK by me. I was just a kid at the time and that “Official Rifleman Rifle” cap shooter under the Christmas tree was just perfect! Killed a lot of redskins and rustlers with that rifle and my “Fanner Fifty” d;^)