the o-ring on my distributor leaks like a sieve, and remembering someone say to get the brown one from GM i went to the pontiac dealer. the parts man spoke very indistinctly, and seemed confused, eventually allowing as how he had a listing for the entire distributor but nothing for the o-ring.
i went to the chevy dealer across town, and the young guy had the same problem, but the older guy went to the racks and returned with a little box full of brown o-rings, didnt even look them up.
$1.96 each, part # 10477565, description "seal".
i bought 2.