Todd: Congratulations on your purchase and welcome to the wonderful
world of musical boxes.
Qualite Excelsior was used on musical boxes sold by by Charles &
Jacques Ullmann of Paris, France; thus the 'U' on the tune sheet. They
were in business between 1881 but not after the start of World War One.
That would date your musical box between 1881 and about 1909.
As to repairs, maybe start by getting a book that describes how
cylinder musical boxes are put together. That might be enough to
understand how to fix the stop knob you are missing. Along with that,
if there is a musical box collector in your area of Ohio, they may have
a similar musical box movement that you can examine up close and
personal to see how it works and 'copy' the missing knob.
I assume it plays; try to get it to stop at the end of a song so that
the cylinder pins are not pushing up against any comb teeth -- leaving
it stopped that way for a long time can cause teeth to bend up out of
alignment. You can use a rolled up small piece of paper inserted into
the governor fan to stop it, as a temporary measure, right at the end
of one of the tunes so you don't have to let it wind down completely,
until you can fashion a replacement stop arm.
Meanwhile, enjoy the music!