"Rochester Optical Company" holders for glass plates.
Although plateholders were not designed to hold film, "film sheaths" were subsequently made to allow its use.
If the interiors of your plateholders have the general appearance of a film-holder interior (i.e., a flat metal back with folded-over edges on the sides, as you describe)--then almost certainly they have film sheaths installed, and that's what you're seeing.
The sheaths are meant to be loaded with film first, then inserted into or removed from the plateholder just as a glass plate would be. Most of the later plateholder designs have a leaf spring inside the wooden frame--at either the top or the bottom--to provide tension to hold the plate/sheath in position. By pushing inwardly with a fingernail (pushing lengthwise, on the opposite end of the plate/sheath), thus compressing the spring, you can get that end (the end you're pushing) to clear the wooden frame. Then you just lift it up and out.