Universal Camera Corporation launched their Univex Mercury CC model in 1938, along with their proprietary #200 series films. It’s basically 35mm film, but on an open roll. The manual shows it having paper backing on the ends only, a bit like 220 film, to protect it from light while loading. Trouble is, the backing paper is perforated, which rather defeats the object.
The camera is often described as ‘half frame’ but that’s not strictly true. The image size is 25x19mm in portrait format, so it will just touch the perforations on both sides of 35mm film. Each frame winds the film on by 4.5 perforations, more than half of the 8 perforations for a full 35mm frame. The frame counter goes up to 36.