In Sergeant Friday's voice-over narrative at the beginning of the show, he states the date is Tuesday, April 2nd. Jack Webb was born on April 2, 1920. That, ironically, was a Friday.
In 1975 it was disclosed that the LAPD Undercover Intelligence Unit had compiled more than 2 million case files on unsuspected individuals during the 1960's and 1970's. Much of the spying and group infiltration was directed at anti-war groups and various civil rights organizations. I.E. the womens movement, gay movement, migrant movement It had been going on at the direction of at least three different police chiefs between 1950 thru 1980.
This is one of the few episodes where Joe Friday is not wearing a gray jacket and black slacks. Bill Gannon always wears a gray suit with a white shirt. Their attire was kept static strictly for the sake of continuity.
"D.H.Q." in the title stands for "Divisional Headquarters".