k is the inner buckle, d is the diameter
A fire hose is a hose used to carry flame-retardant liquids such as high-pressure water or foam. Traditional fire hoses are lined with rubber and covered with linen braided fabric. Advanced fire hoses are made of polymeric materials such as polyurethane. There are metal joints at both ends of the fire hose, which can be connected to another hose to extend the distance or to a nozzle to increase the liquid injection pressure
It is not said separately, it is such a representation of KD 65 (50), KD refers to the caliber representation of the fire interface, and the number behind is the specific caliber size