Hi Leigh,The shiny rectangle is as you say just the release for the viewing hood. The latches are on the top edge and are not too obvious. Once released they allow the back to drop down on a hinge. I have attached another picture showing the back open and the GG partially removed, so that the plate can be inserted. This should make the whole operation clearer.
Ian, I have ordered from AG Photographic and there is a slight delay at present on 9x12 size film. These VN holders seem to most resemble the Linhof type but are slightly longer being 100mm x 150mm. I was thinking that I would use some thin rigid plastic, slightly thinner than the original glass plates to line the holders and ensure flatness.