Well thanks you guys I appreciate the response. I bought this car in the early 80’s as a basket case and I am not sure if the cap is original. It looks like the one that Moss sells with the rubber ring that fits late tr3. However, the cap has this little vent fitting that comes off and perhaps 1/8 opening at the end. It might be a tr4 cap I do not know. I did notice that a late tr3 cap and a tr4 cap from Moss use the same part number. I am just trying to find a place to vent this thing and I think that Randall is right it is really unnecessary. It is just one of those things I want to know. The tank also has the normal vent, so I guess, I might have a vehicle that has both, but again I am not sure. I will take some pictures. The part that makes me feel the cap could be wrong is that it takes some work to get it through the rubber ring and body hole with that little vent collar and then into the tank.
Steve