Looking at my slightly random selection of contact printing frames, I think one of them might be designed for plates - but I am not sure. One says "for film" on the back. Was it simply the case that ones for plates were just exactly the size to hold the glass? or did it fit under the front glass? I have never seen a half plate frames for sale, but on the other hand the non standard size ones I have seem pretty random.
The thing is I would like a frame for printing half plate glass plates and wonder if I will find one in a reasonable timescale. Cant stick a plate under the glass in any of my frames - too tight.