Jacek,
Yes, the original seals were cork. The Filling Station in the US are able to supply these. Most parts suppliers offer replacements in Neoprene rubber though, so it might be easier to get those.
I bought rubber ones off Ebay UK.
I think Chevrolet switched to rubber in post-war production as well (?)
Alex
Cork:
http://www.parts123.com/parts123/yb....Z5Z5Z50000014x
rubber:
http://www.oldchevytrucks.com/cart/p...7&i=10275|||||
https://www.chevsofthe40s.com/detail..._8_Pieces.html