The polaroid film is held very close to the 35mm film plane, so a piece of high refractive index glass provides the correct focus shift without needing a fiber optic light pipe like other 35mm polaroid backs. I believe this is the only polaroid back for 35mm ever designed without the fiber optic light pipe.
The polaroid back has a dark slide built-in, so it can be pre-loaded with the single Instax film ahead of time.
The glass is very fragile and sticks out when the polaroid back is off the camera and is easily chipped when inserting the polaroid back on the camera. I don't think I have seen any of these with perfect glass.
The one below is not mine, but you can see a similar chip in the glass.