David,
Here is a link.
http://chevy.oldcarmanualproject.com/parts/1929_54/219.htm
Look at the exploded diagram at the bottom right and you will see "5.555 (4.318) oil seal" I think that is the part, although it looks like it has a lip on it. That is why I think it is the mystery part that is possibly missing something. Perhaps it was originally rubber molded over the metal.
Kent

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