I don't believe that the re was ever a factory mount, just owner or dealer installed aftermarket units. Most all of the wags and cherks came with either an inside mount on the left side in the cargo area or the underneath mount ala pickup behind the rear axle. Could be wrong though, would not be the first time.
A really faded note from the guy I bought mine from said it was a dealer option that had a swing-away carrier for the tire and a mount for the 5 gallon jerry can already on it. I guess they put some kind of stiffener because after 37 years I don't see any major cracking or bent sheet metal. I believe some other guys on the forum have called them "safari kits."
Your wag strikes me as somewhat "de-optioned" because it has no AC, and is is sans roof rack. It may have had both in the past, but we will find that out this weekend. The dual tanks are aftermarket, not sure if there was ever an option for them. However, yours has the swing away carrier because the aux tank takes up the spare tire spot behind the rear axle. Mine was the same way when I bought it but i removed the tank due to some complications, and am now running a stock fuel tank.
I saw a J-truck with the spare hanging off the grill once. But i think that was less dealer option and more ghetto-tech. I mostly think that because it was mine and I broke the tire carrier and i was not about to leave a tire sitting out somewhere...
There's a '71 Wag for sale here in town that has one, but the dude wants kind of a lot for it, and the body is pretty banged up. I need to find a parts truck to finish out all the little stuff I want to do, but she's taking the girls to school every day!
I can't get the spare under it, probably because of wider tires, and I don't want to run around too far without on... May have to modify the rack underneath (hit it with a hammer until it bends enough to fit around the tire!), but I'd sure like to have one out back where I can reach it more easily.