It's a very fat o-ring. more like 3.25 x 20.5 by the convention you're using.
The black o-ring in the pic actually looks pretty good after I boiled it, smooth with no nicks, not flattened or crushed. It was made with some good rubber in it's day. This o-ring came from the bad R4. There are 2 more in the good R4 which is in the car being pressure tested. They are holding pressure but I'd take them out to let them stretch out a bit then inspect them and put them back in if I can't find exact replacements.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked