Gas caps are vastly annoying. My 1970-71 parts catalog has the diagram and the index entry calling out section 14-86-35. There is no section 14-86-35; its skipped right over on one page. So my massive (and NOT inexpensive) parts catalog can't give me the part numbers for fuel caps on my car.
Sucks.
The cap that came with my '71 car (already a chrome replacement) was stolen. When I bought a replacement the parts guy asked if I had ECS and verified it, then gave me a vented cap. He didn't steer me wrong on anything else but I have no idea if the vented type is 'original' correct for ECS.