Hi Peter,
I think you may have misread my post. I had already acquired the changing box in 5x4 format and was looking for a Marion Soho camera in this format to go with it. As luck would have it I found such an item at Photographica a couple of weeks ago with a jammed shutter which I was fortunately able to unjam. The whole set up has now been cleaned up nicely and is in full working order. See pic.
However I would be very interested to know if your fathers changing box carries any markings and if so what. I may be interested if it is in 5x4 format and has the film rather then plate holders ('septums') inside. There should be 12 of them. If they are film septums they will just allow you to slide the width of a piece of film into the turned over edge. If they are for glass plates they will be a good bit wider.
Thanks for responding