All those that responded to the Stella music box winding problem are
correct: it is a flat wound spring over the winding shaft and anchored
to the main frame.

The trick is making that spring.  What I have done three or four times
in the past is to go to a car dealer and check whether they have any
old dipsticks from earlier model cars.  Those dipsticks are almost
identical to the original spring.  You may also find a few at an auto
graveyard.  

Then you would proceed to use a shaft smaller in diameter than the
Stella winding shaft to wind a new one in a lathe.  But you must keep
good tension on it or it will come out oversize.  You will have to
experiment a little to get a nice fit, but you will get back to what it
was originally.

Ralph J. Schultz