I have no original cap for that tank, the truck didn't have one when I bought it. It seems to be an aftermarket saddle tank, 18 gallons on the drivers side, the neck comes up to a lockable door in the bedside about a foot behind and below the normal tank location. The neck is smaller in diameter than the one on the behind-the-seat tank. This is on a '71 C20.
I bought what is in the link below to use while I search for something correct (just a push-in rubber valve cover cap, but it doesn't seal as well as I'd like):
https://www.amazon.com/dp/B00062ZCSW...v_ov_lig_dp_it
The smallest cam-style twist-on gas cap with the proper style lugs that I have seen is 1-3/8, I need 1-1/4 diameter. It needs to look like this one, but smaller:
https://content.speedwaymotors.com/P...64b7982d81.jpg