the doughnut is part of a mount that goes through a hole in the stand under the reservoir. if all you have is the doughnut, you are missing the other half of the mount.
the nipple on the bottom of the tank goes into the grommet, and that grommet is mounted in the hole in the stand

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John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 560SL convertible
1987 190D 2.5-5SPEED!!!

1987 300TD
2005 Dodge Sprinter 2500 158"WB
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!