Here are the two most common types of AD heaters.
http://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754
The recirculating motor does not extend through the firewall, so it and the rectangle shaped hole in the passenger inner cowl are covered with steel plates. Here is the exterior one.
http://www.flickr.com/photos/advance-design_parts_co/8129182431/in/set-72157631867396207
Here is the interior one.
http://www.flickr.com/photos/advance-design_parts_co/8130538304/in/set-72157631870734114
Here are all the parts needed to install the Air-Flow this is a link to a series of NOS and originals.
http://www.flickr.com/photos/advance-design_parts_co/8129413707/in/set-72157631868452754

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