Float bowl gasket for the holley 1904, or 1920 carburetor. Gaskets are coated with black nitrate, which helps prevent leaks through the gasket.
If you are leaking from the float bowl doubling the gasket, or using a thicker gasket is not the solution. The carburetor body is probably warped and needs to be repaired. See here for 1904 technical information
We also have the old style cork gasket
, which can help with those warped situations.