It should be a paper gasket on the base with a cork/ composite/rubber ring around the cylinder spigot. Depending on how long it has been running with the cork base gasket, you may find that the cylinder has a ridge worn by the rings at the top of the cylinder which the rings will hit if you fit a paper gasket instead of the cork and effectively lowering the cylinder!
It may be very slight, but it could break the top ring or create a bit of a clatter, as I discoved when I replaced a cardboard base casket that someone had fitted with the correct paper one. If there is a ridge, get it honed or bored to remove it.
Dave
