'68 Slant 225 Plug Tube Washers?

The counterbore at the top of the spark plug threads in '63-'74 heads, together with the aluminum plug tube, serves as the plug gasket. Adding another plug gasket makes the plug run hotter (increasing tendency to ping for any given spark advance) and also provides a nice ring-shaped trough at the bottom of the plug threads for carbon to build up and cause problems. See here for details.