Gas cap for my reserve tank.

[PbFut]

I went through similar problem a few months back With a original cab and ax tank on my 72 and an unknown aux tank on passenger side. Since it is a 72 GMC C25 no vents in the system and I needed vented caps. C10s have vented systems and all the caps are non-vented. There seems to be a difference in the depth of the flange on the neck, presumably to avoid the wrong cap from being used. Problem is that that when the tanks were added, venting was not always on the tank and a vented cap was used and then the mismatch of caps occurred. Todays parts guys only know how to find a part with a make model and year computer filter. So you will probably have to compare 1970 and 1972 C10 and C20 caps to get the right one. If you need to make a non-vented cap vented because of the neck flange, you can bend the 2 keeper tabs back on the inside of the cap. The metal ring will release and the plastic assembly will come off. Remove the heavy spring and reassemble. Doing so allows venting without any gas spillage and eliminates that puff or suck of air when you open a non-vented cap on a non-vented system. This also will reduce stress on the fuel pump. as the non-venting of a system will stay at ambient pressure. Good luck with your search.
Oh, and yes, Napa was finally where I found the cap I needed.

After posting that, it occured to me your filler is probably behind a door in the side of the truck with the basic looking cap with 2 tabs on it. I would look for a car that had the filler behind the license plate. The Riviera or Impala comes to mind. I think all those were vented caps as they were built pre Canister EVAP days. I doubt you have a vented Aux tank.