What is the black square on the back of my Olympus OM-20/OM-G?

Ok so I've had my OM-20/OM-G (I have the OM-20 if that helps, I'm aware they were marketed under OM-G in america) for a few months now and its great. Just wondered about the black square panel under the view finder on the rear (it's approximately 1.5 inch X 1.5 inch), it looks like a frame of some sort, maybe its to slot in details about the film? If you have any idea please let me know, thanks.