A modern version but nevertheless relevant to all my days in the navy using communication circuits [1953-1983] particularly on 'CW' circuits, and as a submariner, where unlike those in general service we used CW for much longer to clear and receive our traffic, such as it was. As a result the 'AT' Zulu Op SIGS {chief of which at that time was ZBZ] tended to be a more surface fleet "short cut" leaving the "fish heads" with the 'CW' Quebec Ops Sigs like QRK really meaning the same thing as ZBZ.  Anyway, a ready reference should you require it !