Something else I thought of:
-Since you likely do not know the entire repair history of your horn....being from the 20s and that.... You may encounter glue. If you do you will need an alchohol burner....Gently heat the outlside of the pad cup and the pad will fall out<this is after you removed the button>.
The glue with either be stick shellac or hot glue.
I believe cybersax.com has some great articles on snap pads.