Back in the day, code words were used when requesting mutual aid from neighboring towns and cities. This was done to verify the authenticity of the requestor. The concept was first proposed at the 1950 conference of the North Carolina State Firemen’s Association. And enacted that year (I believe, need to check).
By the time of the 1953 conference, the code word was in-effect and members were asked by Statistician Albert Brinson (yes, related) to suggest a new word for next year. “Brock” was chosen, in memory of the great Sherwood Brockwell, the state’s first fire marshal (among many roles), and who passed away that year.
Recorded in the proceedings for the 1953 convention, President “Moon” Wyrick declared: “Whenever your Chief or whomever you designate, the Mayor, Board of Commissioners or Board of Aldermen, […] need mutual aid one among another, [they] will use the signal ‘BROCK’.”
Sources: Proceedings of the North Carolina State Firemen’s Association, 1950, 1953.