These are tricky. I can say with fair certainty that similar parts do not exist on a 1938 ¾ ton Chevy panel, having had mine in pieces before restoration. GMC certainly had some variations in parts/style; the 37 and 38 years might be slightly different, and most importantly, if I recall correctly, your panel is a 1-ton or 1.5 ton–that is probably the most important factor; there are many variations between truck sizes. All that being said, I will venture a few guesses.
I have no idea what the round canisters might be. The previous post is as good an answer as I can come up with. The brackets near the canisters–if they are 3/16-1/4” steel, they might be part of the rear bumper hanger system (there are a couple variations for the panels), or part of the spare tire hanging system (again, several variations).
The oval washers in the plastic tub, seeing that there are about 9-10 of them, might be body to frame washers (the Chevys have a rectangular type); they seem to have about a ½” hole, which is about right for frame bolts. They might also be part of the bed mounting system/cross pieces for the wooden bed of the truck. I don’t know what the 90-degree angled brackets on the plastic container are–they don’t seem heavy enough for frame, but perhaps hangers of some sort for the fuel tank or other system (exhaust, battery), or body to wood bed again?
The brackets on either side of the gas pedal–are they channeled on the back side? If so, they almost look like part of the door/window regulator system. If they’re not channeled, not sure what they might be. The felt looking thing next to them seems to be some sort of pad; with the hole size, etc., it almost looks like it was used as a pad between body/frame/frame washer–Chevy has rubber ones. Perhaps a pad for a tank mounting strap? Is this the only one, or are there multiple ones? If there is only one, my theory above is probably wrong, but with the hole there, it must have cushioned something of that size.
Sorry not to be of more help–those are my best guesses at this point. One other thought–when I got my panel in boxes and containers (like you), I discovered that the previous owner had collected all sorts of parts and POTENTIAL parts. A lot of these “potential” parts either did not fit (they were from a ½ ton or a 1.5 ton) or they weren’t parts at all, but stuff that he had collected along the way and saved for one reason or another. I discovered this the hard way, trying to fit several of them in all sorts of places. When the truck was finally all back together, there were a bunch of these “parts” still around, but the truck was complete! So some of these things in your containers might not have any actual connection or application to your vehicle.
Good luck!
Jim
