Were 16-inch/50 guns already available for the cancelled American ships?

Didn't the Lexington and South Dakota designs already have 16-inch/50 guns?

Those earlier guns were a different design from the later 16-inch/50 under discussion, which depended on subsequent metallurgy and gun development.

The choice of 16-inch/45 guns for the proposed early-1930s battleships depends on which 16-inch/50 design is meant. Successive gun designs can share a nominal calibre and barrel length. Later metallurgy and development separate the later 16-inch/50 from the weapons intended for the cancelled Lexington and South Dakota designs. The hypothetical early construction program therefore retains the 16-inch/45.