Hint 5

The fact that the displacement was always to the left (in the S hemisphere) is the clue: this is a Coriolis effect. The designers of gunsights were well aware of Coriolis deflection and had carefully built in a correction for it - assuming that naval battles always took place near 50$^\circ$ N! At 50$^\circ$ S the deflection is in the opposite direction and so the shells missed by twice the Coriolis deflection.