That seal is a Best Gasket seal, as I had mentioned. They are made of a tough modern material.

This guy can probably sell you an NOS seal if you prefer:

http://www.gasketcity.com/

I have bought gaskets and seal from him and he also sells Best gaskets. I have an NOS seal in my 216 that I bought from him. It doesn't leak at all.

1951 GMC 1 Ton Flatbed -- It is finally on the road and what a great time I have driving it!
1951 1 Ton Completed

My Chevy Master 4 Door is on the Road!