Chester is shown reading a book, perhaps a 1957 interpretation of the 19th century bit of popular media, the "Dime Novel". It's something called "Cowboy THRILL Magazine" with the subhead "Real Stories of the Old West". Any time a year is expressed in Gunsmoke, it is perpetually in 1873 or 1874. The anachronism here is that the term "cowboy" would not be one used then for anything heroic, and as for the "old west", wasn't 1873 just about how "old" the west can be?