Allen,
As Dave posted:
"Although a lot of rebuilt distributors come with an O-ring in the top groove there is not suppose to be anything. That Groove is an air gap to prevent oil from wicking up between the dist housing and the block.
With an O-ring in that groove the distributor will have a hard time rotating with the vacuum advance, IE: not at all.
I have at least eight 235/261 distributors and none have an o-ring (as they should not). All were collected more than 15 years ago.
