Joel:  If I had one bad disc in a lot of good ones, I would just
discard the clinker.  If the steel ones play properly, all the critical
alignments and clearances must be okay.  Zinc, being very soft, wears
quickly, and I would imagine that the projections have just been worn
down so far that they do not engage the star wheels deeply enough to
turn them through a full arc.

About case restoration:  this is always a controversial topic.  Strict
conservators will generally argue against a complete restoration,
however well and professionally it may be done.  Musical box dealers
and collectors usually take the view (as do I) that any restoration
(case or mechanism) that is properly done by a qualified expert and
that brings the item back to originally manufactured condition is
legitimate.

You are quite right when you say that beautifully restored boxes have
no trouble finding a buyer.  I stress, however, that the work must be
done by a qualified and experienced restorer who will use appropriate
materials and who understands what the limits of authentic restoration
are.  Boxes that look as though they have been dipped in liquid plastic
are particularly offensive.

Joe Roesch