Thanks for that!
I tried a couple of pictures but the entire thing came out white, no black frame at all. Yes, I pulled the dark slide out AFTER I'd mounted the back. The manual says white means overexposure but I would think that the black outline of the 6x7 rectangle would not be white. The film is old, does that mean it is fogged to white or was exposed? I guess I'll need to try a new Fuji Instant pack once I confirm that the camera is working (and I know how to work it) with conventional film. Too bad, I thought the Polaroid would be a quick check. I really should have thought to try it with the original owner to see if he remembered how to use the Polaroid back but I wasn't thinking at the time.