The Buick Master Parts Book shows it is a Fuel Tank Seal, GM part no 1369281. This part number was used on '65-67 LeSabre/Wildcat/Electra 225 cars.
Here is one that shows up in a Facebook post from 3 years ago:
Some more GM parts added this last week; 3818683 6.164 bushing 2 1999437 2.727 bulb 9005ll 97214984 heater 25036510 1.15 switch d1878 8616150 4.29 537181 3.431 bushing 541461 0.858 seal 3...
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