Where would extra F3 displacement go?

If F3 came in below the treaty limit like O3, could the unused displacement buy more armor?

Yes. The most plausible use would be a uniform 13-inch inclined belt over both magazines and machinery.

Any unused displacement could strengthen F3's belt protection. The proposed change raises the uniform 12-inch belt to 13 inches at the same inclination. That would match Nelson's machinery-belt thickness while remaining below its 14-inch magazine protection. Substantial internal and deck armor would remain part of the design.