Sounds like you don't have FTVC, or someone has removed it all. As mentioned, if this is a 49 states car, it will not have FTVC, and would come with a vented cap. The vented cap has an internal labyrinth to block liquid and pass vapor. Something you could duplicate if you have a sealed cap that fits.
Exactly what is different from the '74-77 picture in the parts book? Your response has been vague so far.
Do not confuse the fuel return line with a vent. Significant parts of the plumbing in the picture will be missing if no FTVC. You will always have a 5/16" fuel feed and 1/4" fuel return. The filler will have the filler hose and the burp line that allows air/vapor to escape while filling.
Yes, can be hard to find a gas cap for specific years. I'd look up the part number in the Jeep factory parts manual and search for that. The aftermarket listings are rife with errors, and the PN cross-over is the least ambiguous method. You can also change to filler parts from a different year, with a cap to match the new parts.
Can you put your year/model/equipment in your signature file? You found it when you added "Stoffregen Motorsports."