1914 buescher trombone parts

[vintageinstrumentfan]

I'm not necessarily sure it this is the place to put this because I'm new, but i have a buescher trombone from 1914, and i have almost every part but I have been looking at pictures of it and realized there is a missing part. I can't tell if it's a coin or something, but I would love to know if anyone knows what it is. thanks! if you look it up it's at the end of the slide.