After filming wrapped the studio received a letter from Anthony Caruso, who played Bela Oxmyx. It was a letter from "Oxmyx" thanking the crew of the Enterprise for creating the "syndicate" and things were proceeding nicely on Sigma Iotia II. As he goes on in the letter, it is now the 1950s and he is sporting a crew-cut. He also mentioned wanting to visit Las Vegas, remarking "It seems like my kind of town."
This is the only time in TOS in which the Enterprise's phasers are used to stun, and not to kill, destroy, or damage. It is also the only time the Enterprise's phasers are used for a wide proximity shot, as in this case when they are set to blanket a one city block area around a central point in order to stun a dispersed group of people.
The car that Kirk drove to "put the bag on Krako" had a V-12 engine, as a V-12 emblem is seen on the radiator. It was a Cadillac, probably a 1931 model. Note the winged radiator cap, which Cadillacs of that vintage had. It is a nod to Chicago crime boss, Al Capone, who had a 1928 V-12 Cadillac. It also represents the only time that a member of the Enterprise crew is seen operating any kind of land vehicle within the original series.
Marvel Comics published a sequel story to this as part of their "Star Trek: Unlimited" series. The story, "A Piece of the Reaction", featured the crew of the U.S.S. Enterprise-E (from Star Trek: First Contact (1996)) returning to the planet to discover that its society had gone on to model itself after 23rd century Starfleet, thanks to the communicator that McCoy left behind. The planet is now led by the tough kid Kirk and Spock met in the street, who wishes to hijack the Enterprise-E and finally gain command of a starship, just like his idol, James T. Kirk.