It is my feeling that the original black composition valve seats
of Ampico A and B unit valves are a better choice than metal.  The
"Bakelite" composition of the valve seat does not react with acids
in the leather valve.

All leather contains a percentage of acid used in the tanning process.
Small amounts of this acid can react with bare metal surfaces, unless
the surface is first protected by a coat of clear lacquer, or corrosion
preventative paint, such as Rust-Oleum.

Bruce Clark