I agree with Stuart. The 1947-54 Chevrolet Truck Shop Manuals and the 1947-54 GMC Maintenance Manual (which has a full page diagram of the distributor, with all parts labeled) do not make any mention of a distributor gasket, seal, or o-ring. There is no distributor gasket, seal, or o-ring list in a Master Parts Catalog (until 1957, for a V-8).
However, some people have posted that there was an o-ring.
