Sorry if this is classed as 35mm..I guess it straddles the two! I have a few dozen of these inherited from my dad. They used 127 film to shoot 4cm x 4cm transparencies which were mounted in thin-bezel 2"x2" mounts and could be projected by regular 35mm projectors. Here's a photo showing the difference between super slides and regular 35mm: I guess they ultimately died a death because they straddled a no mans land between the convenience of 135 and its 24-36 shots on a roll and 2¼" square aka 6cm x 6cm. Compared to 2¼", super slides had the same drawbacks compared to 135, ie greater expense and fewer shots on the roll, but 2¼" gave even higher quality images.