I suspect that the glop you cleaned off had to be either old grease from
the spring and motor, or burned shellac.  If it was burned shellac, it
would explain why it came off with warm water. The heat will loosen and
remove it from anything.  Did any music box companies coat the weights
with shellac to protect them like pipe organ companies did with the
unburned shellac on their organ pipes?  I wonder.  I do not normally work
on music boxes.

D. L. Bullock    Piano World    St. Louis