Considering this, what is a guitar strap button?
This refers to the metal button that is typically screwed into the body of an acoustic or electric guitar onto which the strap is attached during live performance where the player must be (or chooses to be) standing. There will be two strap buttons on a guitar, one for each end of the strap.
Keeping this in consideration, what kind of guitar strap did Eddie Van Halen use?
EVH guitar strap