Immediately after the end of WW2, as the age of fast-moving jets dawned, most militaries around the world concluded that traditional anti-aircraft weaponry was rendered completely useless. Bombers flew higher and ground attack aircraft faster than traditional hand-powered AA guns could contend with, so most anti-aircraft weapons were withdrawn from service and destroyed for scrap while efforts were made to develop the first Surface-to-Air missiles to counter the threat of nuclear bombers.
By the end of the 50s however, the widescale introduction of helicopters onto the battlefield forced militaries to re-think their abandonment of battlefield anti-aircraft weaponry. The anti-bomber SAMs they'd been developing were not suited for active battlefields to shoot down helicopters, so some nations developed new weapons while others re-introduced some old favorites.
M45 (M55) Quadmount
The infamous "Maxson" from AS2 makes a return. The M45 Quadmount anti-aircraft turret (mounted on an M20 trailer to create the M55 Anti-Aircraft Weapon System), unlike most WW2-era anti-aircraft weapons, was never truly withdrawn from service in the US military. It saw action in Korea and later Vietnam primarily as a fire support platform against ground targets with its four powerful M2HB heavy machineguns. In Europe it was one of the only anti-aircraft weapons ready to clear the skies of Soviet helicopters.
The ZPU-2 was developed immediately after WW2 by modifying the KPVT 14.5mm heavy MG into a light anti-aircraft weapon that could be easily carried by troops or mounted onto light vehicles when needed. Despite its larger caliber than the M45, the fact that it has only two barrels makes it roughly balanced with the American system. The ZU-2 as seen in-game was a lightweight modification for airborne troops that also saw widespread service.