I think I have identified the power switch for the Beta series amps. I've ordered the part so I can verify fit and function. If anyone thinks it will fit in another amp, post a reply here. The hole in a Beta amp is (inches) 0.550 wide by 1.125 high.
Carling Tech part number LTA201TRB/125N
The Mouser part number is 691-LTA201TRB/125N and their website says they have several hundred in stock.