Wilson Foundry Ford Blocks
Wilson Foundry made some of the Ford blocks for the GPW. They have the Willys 638632 casting. Three other characteristics appear to differentiate the Wilson from Ford castings. They are the shape of the boss, the "W" in the serial number and the date stamped on the oil pan flange. Below are some examples.
GPW 5127 638632 casting (Rob Lihani's excellent early GPW with matching #'s on MB frame)
GPW 99786 638632 casting
GPW 103207 638632 casting (ICO John Stiles)