There are several versions of the recirculating heater. The two most common are the early like mine which are up to mid-51. They are slotted to accept an aluminum plate with Chevrolet or GMC see below:
http://www.flickr.com/photos/advance-design_parts_co/8129304429/in/set-72157631868452754
Then there is the later version (this is my complete NOS in box) which is the decal style, mid-51 through 55 Series 1.
http://www.flickr.com/photos/advance-design_parts_co/8129330842/in/set-72157631868452754
There is also a GM Canada version that is a different color/decal style I will
post a picture later when I can find it. There are also some that were used in passenger cars that have a sheet metal enclosure added to direct air flow.
http://www.flickr.com/photos/advance-design_parts_co/8129370166/in/set-72157631868452754

1950 Chevrolet 3/4-Ton 3600
More pictures on Flickr