Yes it is spring loaded, momentary. So probably ON OFF ON. The fact that it is recessed is what made me think it was factory.
Actually the car did have a radio at one time. When I look up in the drivers wheel well there is a bracket still there and the antenna hole had been welded up prior to a repaint. So that may have been what the switch was for if it wasn’t factory.
Here’s a pic looking up inside the wheel well where the antenna would have been.