Thanks for the link Leigh on the 9x12 plate holders. I need to read this closely to check for possible alternatives. The edges of the VN plate holder are plain as in the pic attached. I also have an old Soviet grooved plate holder which appears to fit although my double plate holders are too thick for the back to close properly. I have also attached a picture of the back which shows the latch arrangement along the top edge. Unlatching allows the back to drop open, the GG to be removed and the plate holder to be dropped in. Closing the back causes to springs to clamp the plate holder in tightly and the latch stops the back springing open.