This certainly is a new one on me.  Are the long slits above and to
the right hand side of the note holes, a kind of tracking device (the
bar does not seem to have the usual holes) and since there are no
tube nipples, was there a plate which mated with the back of the bar
as shown which had connections for notes and the "tracking slits" if
that's what they were?   Just a thought.

Paul Morris