Detectives and plain clothes officers were not required to carry the standard 6" barreled revolver on duty
I'm quite certain that LAPD dicks were required to carry the six incher in the early 60s. My LAPD buddies complained about it all the time. By the time that version of Dragnet (66?) and Adam-12 (later) were on the air, policy had changed.
But I'm just not sure what they did in 1949 or when the "all must carry six inch" policy came to be.