For larger AA weapons, such as these M39 37mm cannons, flatbed cars were used as a platform.
When not combating aircraft the armor panels of these flatbeds were raised, to protect against ground fire
When guns were firing the panels would be lowered as seen in this photo.
This type of folding side box for Russian AA was known as the BP-34 type and came into use late in the war
Oddly enough, functional design for the BP-34 mimics another anti-aircraft weapon of the period.
The German Flakpanzer IV "Mobelwagen"