Your casting number confirms the rear came from a 1940 Chevrolet 1/2 - 1 ton truck. My books show this a a one year rear...
The casting number that looks like it's a screwed on tag is the date code, so if they used letters for the month than A = Jan, if they used numbers than 4 = Apr. The "zero" would be the year last digit which is 1940, the only zero year that used that casting part number.
Mike B
