Cork flat ring. It is not the best fit, but it does seal.
Thanks Russ, I'm assuming that's a DIY item?
The gasket is part of an engine rebuild kit. I have not found it separately yet. But you can make it. The cork material is very thick.
It may be with the valley pan gasket. Give Best Gasket a call, they may know.
As seen in the picture below there was a cork gasket between the PCV Return Pipe & Connector and the valve lifter compartment cover. I have a new gasket for $5 + s/h. Under the long screw bolt there was a large washer 1 13/32" wide, 7/64" thick, with 9/32" opening. These were on all 1963 & 1964 Cadillacs.
The Master parts books also lists a smaller washer (9/32" x 5/8" x 1/16") for 1949 to 1962 and 1965 to 1966 Cadillacs, however, I have seen them on very original 1963 and 1964 Cadllacs including an early production relatively low production 1963 I am parting out.
The picture below shows the PCV return pipe, bolt, pipe to compartment cover gasket, smaller washer, and standard larger washer. If interested in the gasket shoot me an email to firstname.lastname@example.org.
The gasket is though Best Gasket. PN 1270 Ventilator tube. They do not sell through their factory, but through established sellers. Look at their web site https://bestgasket.com/distributors/#cadillac for a seller and see if they have it or can get it in.