What Furious reveals about the origin of the 18-inch gun

Was Furious's 18-inch gun the same weapon as the one considered for later G3 and N3 designs?

No. Similar broad parameters did not mean the same gun: calibre lengths differed, and the gun's power had to be reduced for Furious's framing.

Similar design parameters do not make two gun projects identical. The comparison with later G3 and N3 work distinguishes different calibre lengths. Even the power associated with the shorter gun was too great for Furious's framing and had to be reduced. That mismatch supports the interpretation that Furious and the gun were not originally designed around one another.