I think it may be for some type of folding plate camara, ala voigtlander avus or similar. Not sure of size, and there were many styles without a set standard for the fitting, so you might verify holder dimensions with someone.
I say this because of the thin metal ridges along the sides with wear on them, suggesting this slides into a narrow metal track which is typical of plate cameras of this type (Voigtlander Avus, Bergheil, Zeiss Maximar, Kodak Recomar, etc).
I have one of these, and it fits my Kodak Recomar 18 cameras just fine. In fact, I wouldn't mind picking up a second one.
Does it have both of the film masks with it? If you lift out the actual holder portion, there should be a mask underneath it to set the frame size to either 6x6 or 6x9, and there should also be a second mask for the other size. If it only has one mask, which size is it?