Dist. installed with O ring instead of gasket

I had a local shop rebuild a '70 360 that I had slated to install in my wife's '68 Javelin and am concerned with something I found after I received the motor and dropped it back in the car. I was sorting through and organizing all of the paperwork returned to me by the engine builder for the new motor parts used and I saw a slip of paper that reads:

Notice: Use supplied O-ring on distributor only. Do not use a gasket.

This is concerning because previously, while I was adjusting the timing I noticed a gasket at the distributor shaft mating surface. When I asked the mechanic who built the motor about it he had no idea what I was talking about. Since there were no manufacturers identifying marks on the notice slip I can't find details about the O-ring size or placement for proper install. I know that correct spacing and gear alignment is critical to the survival of the distributor and cam gears and using the wrong seal could be catastrophic. If anyone has any experience with, or knowledge of similar circumstances please weigh in with advice. The timing cover is Omix-ADA, and the distributor is a Pertronix HEI.
Any help is appreciated.