This air compressor does indeed belong to a Blitz, it is selectable, and is used for tyre inflation. I don't know if the Ford is version is different to the Chevrolet version, but if different, they are very similiar.
The DUKW air compressor is a much bigger twin cylinder unit, shaft driven throught the radiator, attaching the the front main pulley. They are always operating, incorporating an unloader valve.
Aaron.