That is the the music holder that would fit on a trumpet or cornet as pp and jth have said. Besson were brass instrument makers and I have a Besson cornet that has an identical clip.
Music for marching bands is produced in small books or sheets.
Drill a hole in your computer desk top tambo. Clip your copy in the err... clip, and stick it in the desk hole. That way you'll be able to read text that you want to type.