Funny, I was planning to post about these too.  I had just rediscovered
such a birthday card from about 1955 -- the size of the usual card,
maybe 3/4" or 2 cm thick, with a picture of a bear playing a tuba on
the front.  It still plays!  Well, it does hang up in a couple places
where the belt may have taken a set.  The tone is really quite good,
using stiff wires for comb teeth.  Don't know how the factory tuned the
teeth, but the pitches are fine.

As a boy I also had one of those Davy Crockett ukuleles with a crank
out the side and the same rubber belt system.  A musicologically
significant feature of these mechanisms is that they play equally well
in either direction.  I can assure you that "Happy Birthday To You"
gives some valid chord progressions and melodies in reverse, but
"Davy Crockett" in reverse is worhtless (if I remember right after
45 years :-).

Also as a boy I recall thinking that these "music boxes" would wear out
from the comb teeth cutting grooves into the rubber "pin" projections
-- you could see it happening, and hear the music getting softer after
a few dozen (hundred?) plays.  I think my parents arranged for the
ukulele to get lost before the rubbers wore out, though!

I distinctly remember a trick used in the uke: the accompaniment part
of the comb scale was missing even some diatonic notes, so one wide
rubber "pin" could play two notes of a chord at the same time, a third
apart.  Try that on your Nicole!

I wonder if any more of these survive in playable condition?  I'm
amazed the rubber hasn't turned to goo in mine.  And, knowing myself
at that age, that I never took this one apart to see it work!  Luckily
I could watch the ukulele play through the sound hole.

Mike Knudsen