Some previous owner bodged the cap seal with red silicone. After staring at it for years, yesterday I pulled it loose to reveal that a thin layer of black rubber is stuck win the silicone as is the decrepit layer of fabric (is that supposed to be there?) and another layer of black rubber is affixed to the cap. Also as the first pic shows there is this black rubber ring that came with the car that appears to be the right size to be a replacement seal, although it would need to be glued in after removing the rubber that is held in by the riveted disc.
Your filler neck looks of to me as narrows at the top. Mine is constant diameter all the way, Mine isn’t a federal car thou. (I’d post pic but for us to large for my phone camera)
