Why oversized battleship guns cannot stay secret

Could Germany hide a much larger battleship armament until its ships were launched?

No. The hull, gun mountings, industrial tooling, testing, and transport would expose the project well before the ships entered service.

The proposed deception fails because a ship's barbettes and hull already indicate the size of the armament they must carry. Even before fitting out, large-gun development leaves a trail through specialised tooling, factories, proof ranges, and logistics. Japanese-made 41 cm guns would create additional signals through docks, long-distance shipment, payment, and fitting out in Germany. Even successful secrecy would have limited effect because protection against 16-inch weapons was already within British planning. Discovery would also accelerate the Lion-class programme. Industrial intelligence can reveal as much as a ship's final silhouette.