The reason that everyone is surprised when Dr. Schultz calls Doc "Galen" is that Doc never went by his first name in the series up to this point. He was simply "called" Doc. It was Milburn Stone who decided that that would be his character's first name. He was called Galen by Theodore Bikel's character in season 10's Song for Dying (1965), but never by anyone who lives in Dodge City.
During a dental examination, Doc finds that Festus has 36 teeth, instead of the normal number of 32.
This episode has Doc Adams having been in school with Dr. Schultz. And that is why Dr. Schultz knows him as Galen. But in the radio episode "Cavalcade" (aired 31 January 1953), it was revealed that Doc was a fugitive wanted for a murder in New Orleans and was using a alias. He had killed a man in a fair duel, but as a Northerner/outsider, he was forced to flee.
The piano player in the Long Branch Saloon is seen onscreen, a rare occurrence.
The great majority of references to this title, including at least two books on the TV series, incorrectly include a comma after "Schultz". The title card spreads the words over four lines as follows: Doctor / Herman / Schultz / M.D. The only punctuation is in the bottom line.