Preserving German naval-gun expertise after Versailles

How did Germany preserve large naval-gun expertise after the First World War?

By maintaining the guns of retained pre-dreadnoughts and doing relevant industrial work for foreign customers.

The postwar restrictions did not eliminate every route by which German engineers could preserve large naval-gun knowledge. Maintaining the permitted pre-dreadnoughts required continued attention to their guns, metallurgy, and supporting machinery. German firms could also work for Dutch and other foreign customers. Those practical tasks kept expertise alive without requiring a conspicuous secret battleship-gun programme.