Hi Mark.  I'm no expert, but from your description it sounds like a
similar problem I have had with tin music box disc's.  I believe the
original lettering was done with a form of "Black Japan," which is a type
of paint.  I did some experimenting but came to the conclusion that it's
best left as is.  I, too, used metal polish with reasonable success.
I don't know if you can get it out your way, but the best to use I found
was "DURAGLIT."  It is a wadding type polish; also "AMWAY" paste is not
bad.  Hope this helps.

Cheers¶
Brian Forster