Alfa Romeo produced 28 6C2500 SS chassis during the war. I think Alfa Romeo built at least 6 cars on those chassis for military officials during the war.
Pinin Farina created this car for industrialist Arturo Fumagalli. Front suspension design was licensed from Porsche.
I don't have an absolute answer, why it was finished in 1946. Other than, they waited until the WWII was over?