I think the metal makes a noticeable difference.  Zinc is a soft
metal, unlikely to vibrate much if at all in the treble, whereas
steel (especially plated steel) is more likely to vibrate in the
treble -- not by much, but enough to be noticed.

To test this, try playing a zinc disc on a twin comb 15-1/2" Polyphon
with the lid up, then compare it with a steel disc playing the same
type of tune on the same machine; ideally both tunes should have strong
bass notes and trills in the top treble.

The zinc disc should give a slightly muted and 'mellow' sound; in some
way it sounds older.  It's not that the bass notes play louder, it's
that muting the treble makes the bass sound louder.  The difficulty is
testing "like for like" -- finding a good condition zinc disc won't be
easy.

Would it be possible for a disc maker to make two copies of the same
disc, one steel and one zinc and play both on the same machine?

Jim Weir