The meaning of the number of the number on the bow of Japanese warships in 1938

A letter of the German naval attache in Japan dated Tokyo, Japan 8 July 1938 reports that conversations with Japanese navy officers confirmed that the number on the bows of Japanese ships referred to the flotilla to which the ships belonged. As example 30. Destroyer Flotilla. Lacking a name on the hull was the bow number similar to the tactcal nummer of the flotilla.

Source

High Command of the Kriegsmarine (OKM). Case 36.