Hi,  Although the insides of that piano were probably painted a few
miles from my house, I can't tell you what the paint really was.
But I can say that the name sounds like 'spirit varnish' which is
shellac.  And Spirit Black certainly sounds like shellac made with
alcohol that is dyed black.  I use it on music boxes all the time.
If you use just one coat, it seals the wood and comes off a dull
black.  Add a few more coats and it's ready for French polishing.

Regards
Craig Smith