Whoops: 4 3/4 x 6 1/2 inches.Half-plate is 4 1/2 X 6 1/2, right? If so, I have a Thornton Pickard triple extension half-plate camera in pretty nice condition. I don't have any holders, however. I understand there is a special ebay for the UK? I guess that might be the place to look.
I've always wondered how these holders get separated from their cameras over time.
Sandy: looking at the ground glass panel back of the camera, measure the distance from the left flange (groove) to the right flange (groove) when the film back is in vertical portrait mode.
Now omitting the flanges, measure the same distance without the flanges in millimetres.
If I remember off the top of my head, the distance without the flanges is markedly less than 150mm (144mm to be precise) and 148mm with the flanges.
If this is the case, then you may have to be very careful when finding bookform plate holders...
The only special Ebay in the UK is the one where err, nothing nice happens.