I photographed these Towed 40mm Anti-Aircraft Gun examples at Dubois and the Yuma Proving Grounds. The Navy also uses the Bofors 40mm M1 Gun in its ship defense system.
M1 Towed 40mm AA
NMMV Dubois WY 2025
M2A1 Towed 40mm AA
This was ID’d as an M2A2, but signage at the YPG states M2A1