I just checked the gas tank on my original 1932 Special Sedan to refresh my memory. Actually, there is a fair amount of original black Japan enamel still remaining on the tank. Portions of paint are remaining on both ends of the tank (besides on the top as mentioned previously) and on the bottom of the tank as well. Also, the tank is plain steel and it was never tinned.
The original gas tank on my 1930 Chevy was also painted with black Japan enamel over plain steel. The tank was not tinned.
My '32 was manufactured in the Oakland factory and my 1930 was manufactured in the Kansas City factory.
A couple of filmstrips in my collection show the gas tanks on the vehicles featured as being painted black. I suppose it's possible that the gas tanks came both ways....either tinned or painted.