EDIT: I was wrong, just something to do with instant production/build in CC. First statement still stands though.
EDIT 2: These are some of the units affected, let me know if I forgot any.
M-42 45mm Anti-tank Gun
Zis-3 76mm Divisional Field Gun
ML-20 152mm Gun-Howitzer (Yeah, gun-howitzer is its name in game)
B4 203mm Howitzer
Ordnance QF 6-Pounder Anti-Tank gun (Doesn't seem to affect US 57mm though)
Again, let me know if I forgot anything