I don't have a picture but you should find a sheet metal chain bracket inside the neck of the gas fill tube. From memory it is on the bottom half of the tube about 3" inside. I think it is in the form of a rectangle with a central slot running about 1/2 the length of it. The un-slit end of the rectangle is spot welded or soldered to the inside of the tube and the slotted end faces the gas tank. The two halves of the slit are bent to go around a ball of the chain. You should see it by peering down the tube with a flashlight. In theory to fit - bend up slit tabs, fit chain ball into slit, then bend tabs back down.

I suspect that not many wagons have survived with the chain intact for 45 years. Once the chain is gone the little sheet metal tab tends to get a pounding from the gas spout over the years. The tabs are either likely to be flattened or be sitting in the bottom of the gas tank.

I can get a photo later today if you like. I was planning to fit a chain and got as far as refurbishing the gas cap and buying the chain. I still need to fit a length of tube and chain to the gas cap and mount in the filler tube.