The soviet AA truck is a mobile quad maxim gun. Unfortunatly, it cannot fire at ground targets and is relativly useless against air as well. An extreamly rare sight, as halftracks are just as good against air and are only slightly more expencive - not something you often hear after mentioning four maxim heavy machine guns.
It's low range makes it even weaker against enemy bombers, as they will always get within bombing range before being shot at. The only redeeming quality is it's low cost, but since it's actual use is very limited (low flying aircraft, with a very small attack window), often large amounts of M5 halftracks will be made instead, or even actual flak cannons, as they are usually easy to get as soviets with the added mobility of redeplyment.