IMO That looks like a LED and not a button. It looks to have a red lens, a metal holder, and clear +/- leads for soldering. In fact, this looks like it to me (although I'm not sure if the size is a match):
Keep your 5mm LEDs in place with these chromed metal LED holders. These are handy for projects using individual LEDs as they look sleek, and your LED won't budge. ...