Um, i haven't seen the inside of that amp, but i beleive the button is a common plastic SPDT or DPDT, with the aluminium "button" mounted on the chasis. This is done a lot, from TV sets to industrial equipment.
the button is a rather heavy stainless steel / brass "front end" with a standard plastic switch mounted behind. I use one as a starter knob for my car, bought it at conrad in Germany. (www.conrad.de part n. 706497)