I was at my neighbour's place today, who are even more isolated than we are - they live right off the grid. She showed me a couple of film holders her dad had. He was an avid large format guy. They are old and kind of beat up, but beautiful nonetheless. The dark slide says, "No.3 Eastman Film Sheath 6 1/2 x 8 1/2. Made in U.S.A. by Eastman Kodak Co. Rochester, N.Y. Patented in U.S.A. May 11, 1915".
Each holder has two dark slides. The two sides are held together by brass clips, and if you undo the clips it folds open. I don't know what the wood is, but parts of it look like oak, possibly some is mahogany.
Is this what I've seen referred to as a "plate" size? Is there some reason for that size, or did it just seem like a good size at the time?