You are wrong.
Airfields guns with the single barrel are 37mm (same as on the Ostwind). CVs have some quad 40mm mounts along the side of the tower, and the escort ships have a lot of single 20mm mounts (like you see on the PT boat).
Hard guns refer to hardened positions, armor-covered guns. Therefore a gun without this must be "soft," no?
Not sure it's an official WW2 term, but it's a logical description. That had to call it something.