These are the right size for the old DMG backlight/bivert mod & flips polarity by rotating the film by 90 degrees. You only need this if you've installed a bivert chip to flip the polarity once already. Supposedly the bivert is meant to improve resolution (having installed a couple of biverts previously, I remain slightly sceptical).
Be aware that with the bivert & film installed, the DMG screen will appear a very dark blue (off-black) when the unit is powered off, but the polarity looks correct once powered on.
Only needed one film but doesn't hurt to have a spare. If nothing else the spare could be handy for little science experiments with my son.