The Stella and Mira boxes had a tension spring wrapped around one
of the posts which goes across from plate to plate and is part of
the winding arbor.

It worked by tightening around the post when the spring tried to
backwind.  While it worked well, many of them that we see now are
broken.  You can change it by making a ratchet/pawl setup.

I have some pictures of it on a Mira which I will send to you.

Nancy Fratti - Nancy Fratti Music Boxes
Canastota, New York, USA
http://www.nancyfrattimusicboxes.com/