[cge]

A perhaps similar sort of finder existed on a number of older (eg, first half of the 20th century) cameras, usually as a secondary option or an accessory, for quick shots. They tend to be common additions built into waist level finders because WLFs are slow to use and adding them is cheap; there’s one on the 1939 Praktiflex, as a random example, an early 35mm SLR with a pretty tiny WLF as its primary viewfinder.

Those usually work by having two square windows, the back larger than the front: you know you are looking at them the right way when they line up. They’re very approximate, but they’re meant to be.

Here, the camera has a thickness that is obscured by the face-on photos of it, so I expect it works by a similar principle: if you see the sides of the inside of the screen, you’re misaligned.

[Groxx]

It's nowhere near thick enough to handle the huge variety of ranges shown in https://www.youtube.com/watch?v=duKNwA96J5Y (thickness clearly visible at ~10s)