The relevant 4.7-inch lineage began before the 1930s

When did the Royal Navy begin using the 4.7-inch guns relevant to this scenario?

The specific marks discussed entered service around 1930, while their technical family reached back to a 1918 weapon rather than to unrelated 1880s guns of the same nominal calibre.

The particular marks used in the counterfactual date to about 1930. Their useful lineage, however, extends back to a Royal Navy 4.7-inch weapon used in 1918. An 1880s quick-firing gun is too remote from the design family under discussion. This boundary matters because the scenario depends on modifying a mature contemporary system, not merely finding any earlier gun with the same bore diameter.