Fine, I will try the Moogie character.
The gaskets are a very pliable rubbery substance (probably a plastic of some sort) and are pretty specialized.
They are about 1.25 or 1.5 inches tall, circular, with a top which overlaps the "threaded neck" (to provide a seal) on the plastic tanks of the Mk. II's. Eventually, they break down and fall apart when you open the top of the tank to clean the tank or replace the level sender or gasoline in-tank pump. They provide a seal to prevent leakage as the un-baffled fuel sloshes around in the tank.
I am in the process of putting an '85 Jetta back into service and I was going to open the tank and reach in and feel for grit and use a light to search for water....this is an old car with an unknown history.....with a "biodiesel" sticker on the window....need I say more? Plus, I was going to check the screen on the inlet. Hey, it's easier to do all of this *** initially when getting the car running, than to be sitting along the road wishing I had done it 500 miles later! If I find debris, I simply pull the tank and cleanse it.....the old "lather, rinse, repeat..." thing.
Every car this old which I have ever opened up the top cap has needed a new gasket.....that would be like 7 cars. Plus, I run a high degree of alcohol in the CIS cars (up to 75 %) and the booze is a little harder on the rubber stuff.