This is a perfect replacement for your old timer on your ProPack 100-Series camera (seeing as how the original ones always seem to be missing the battery cover)and has a better layout and bigger display than the factory timer. Fits right in the original location in the middle of the film door and the VHB tape holds it in nice and securely. Good and loud and couldn't be simpler to use, and absolute must for anyone like me who insists on clinging to peel-apart instant fims, or great for monitoring the development of Impossible integral films before you expose them to light. Being N.O.S. Polaroid, I know they'll dry up eventually, so if you're thinking about grabbing one and you're on the fence, just get it- it's well worth having!