Slitherine

The British Bofors 40mm Mk1 AA is listed in the units.csv with a start date 1/9/1942.
It was actually available from the mid 1930s (Britain bought a number of licensed Polish guns before manufacturing their own).
The twin gun was available ca 1/1/1942 and the quad version ca 1/4/1942.

Edit: AP shells were also available, so maybe there should be a switch-to-AT option similar to other AA guns.

Above all this kind of unit should not be able to move and fire, and it should not be able to move more than one hex if not towed. The current stats and rules make them more efficient than fighters.