Where an oversized barbette's treaty savings disappear

Could a battleship save treaty weight with twin turrets on triple-sized barbettes, then receive triple turrets later?

Yes in principle, but the oversized barbette and its machinery remain, so most savings come only from the omitted gun, crew, and associated handling equipment.

The follow-up proposal gives Nelson four twin 16-inch turrets whose barbettes could accept triples after treaty escalation. Omitting one gun saves the weapon, crew positions, breech equipment, and part of the ammunition-handling system. The ship would still carry the large barbette, armored opening, rotation machinery, and a strong mounting or filler structure beneath the smaller turret. The idea remains technically possible, but the weight saved under treaty rules would be much less dramatic than the missing barrel alone suggests.