Does it have a plate or anything on it? There were several non-factory heaters available from a variety of vendors. I have not seen a similar GM heater so would believe it is aftermarket. The value is relative, but by condition I would guess less than $50. Unless it is Art-Deco see picture, or a factory heater with good demand like a Recirculating or Air-Flow, or NOS they don't bring much in my experience.
http://www.flickr.com/photos/advance-design_parts_co/8377831744/in/set-72157631868452754
http://www.flickr.com/photos/advance-design_parts_co/8129448404/in/set-72157631868452754

1950 Chevrolet 3/4-Ton 3600
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