Nelson's high secondary mounts began with a dual-purpose idea

Was a 6-inch dual-purpose gun being considered as early as the 1920s?

Yes. It was an original idea for the Nelson class, but it was not viable, leaving the high-angle secondary mountings as evidence of the requirement.

Interest in a 6-inch dual-purpose secondary battery appeared well before World War II. The Nelson class was originally considered for such a weapon, but the concept was not technically viable. The unusually high elevation of the ships' 6-inch mountings preserves a trace of that design ambition. Those mountings distinguish an early requirement from a weapon system that could actually meet it.