Not to get sidetracked, but these all metal holders are very thin. There's a semi-circular spring on one end to push against the glass plate to hold it in. The ground glass "back" is a similar size to the holders. The back slides up & out and the plate holder slides down to take a picture. Double stick tape was my first thought ages ago. I think the whole camera is light leaky now. I'll just look at it. Use my real cameras.
Ok, looking at it now. The camera suffered major damage in the move. The screws holding the front to the body are ripped out and gone. The lens is a Wollensak made in Rochester, NY USA. The plate holders are stamped BREVETE 500G. Apertures: 1 2 3 Shutter speeds: 1/2, 1/5, 1/25, 1/50, 1/100 B T. All speeds seem to be the same, probably 1/100. It's toast.