First time I've seen that design. What does/did it do? Is it a nightlight? I've seen duplex outlet covers with built in night lights, they have contacts on each side the hit the screws on each side of the duplex receptacle to provide power to the LED light. Hard to do that on a switch because you generally don't have a neutral available on the switch.
What is on the back side?
Any parts or stickers or part numbers?
The window might be for setting a timer.
Or it could be a photo cell override of the switch.