Yeah, I've got some that size. I will probably have 1 or 2 to sell that I don't need.

The bookform back is 255mm along each side. The holders have a tab on each side to slide them into the back and then the back locks them in, so they are not congruent objects with regular holders. It's a pretty clever system. The holders each have manufacturer-placed numbers. The back has clipped corners and the nickel hardware is made to fit around clipped corners. Very interesting stuff.

Depending on your target camera, the easiest way (in my mind) would be to make a frame that fits your camera and holds other backs, including the bookform plate back. That's how the 4x5 was added to this camera by B&J in the 60's.