What you are dealing with is a version of these, common to find them in this or similar color shades. These commonly appear on eBay in NOS condition or good used. Expect to pay in the $100 dollar range for a good set.
http://www.flickr.com/photos/advance-design_parts_co/10089772153/lightbox/Kerosene road flares, several brackets were available, you usually find them stacked on inner cowls or bed sides. There were several manufacturers and at least on the GMC side versions appear in the master parts book specifically the 1937-53 version I have. When I get a chance I will look it up and post the parts/section numbers.
So how popular? Here you can see a GMC with a box of them on the floor, they were in the tool box pan.
http://www.flickr.com/photos/advance-design_parts_co/8130539326/in/set-72157631870734114Here is a bracket on the forward DS bed side, flares missing but you can see the lock down with a small tang.
http://www.flickr.com/photos/advance-design_parts_co/8626465790/in/set-72157631870734114Another type (this version appears in 60's Chevrolet truck material).
http://www.flickr.com/photos/advance-design_parts_co/10092819666/http://www.flickr.com/photos/advance-design_parts_co/8262113533/in/set-72157632220275326In 30 trucks I have seen 4 of these installed from complete to bits and pieces.
And again another that appears in 60' Chevrolet materials. I have found a couple of these in trucks, the are available for as little as $10 near new.
http://www.flickr.com/photos/advance-design_parts_co/10092763184/http://www.flickr.com/photos/advance-design_parts_co/8262113765/in/set-72157632220275326
