distributor

Two o-rings as pictured are correct starting with the introduction of the slant 6 in 1960 model year. As some commenters have posted, most blocks have a 45 deg. chamfer to accept it. I say most because not all did. I've run across industrial blocks that did not have the relief cut in them. If your block does not have the chamfer, or relief cut, trying to use the upper larger o-ring will not allow the distributor to seat properly and would ultimately destroy the plastic drive gear as others have commented.