16 October 1963: ACP 121 Communication Instruction General Message Precedence Categories

In October of 1963, the RCAF received word that GCHQ in Britain was going to conform their signals system to the new U.S. precedent system – as their network was very closely integrated with the American network.

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Hydra Communications - Policy, RG24, 1250-36 Vol. 2, Library and Archives Canada (LAC)