Right you must be plain high, the US have the Halftrack Upgrade that gives you a quadruple .50 cal mount on a very mobile vehicle that does wonders versus Airborne planes and Infantry alike, while the British have the Bofors emplacements which means they don't need a mobile version of an AA gun. Anyway they don't need AA cause the only plane they have to deal with is the PE Lufft Tank Killer which isn't really much to deal with anyway.
Plus the Duster is actually the M42 Duster which was in fact deployed in Korea not WW2.
The reason the Germans have such good dedicated AA tanks is cause the Allies are the only ones with any serious Plane Support anyway