How long had the Crown Colony six-inch battery been in development?

How far back did development of the Town- and Crown Colony-class guns go?

Development went back well before the ships themselves, with the Royal Navy concentrating on getting a triple six-inch mounting into service.

Development of the Town and Crown Colony main battery went back well before the immediate designs. The Royal Navy concentrated on getting the triple six-inch mounting into service. That mounting, illustrated with HMS Belfast, was the central weapon requirement.