I saw a picture of one on the Antique Automobile Club of America.

http://forums.aaca.org/f170/harrison-heater-core-i-d-377038.html

This link will bring you to their forum page discussing the heater. Both heaters shown in that post are gray hammered paint. It is suggested that the recirculating type was used on the heavy trucks while the higher-end heater allowed outdoor air into the unit.