Thanks for the information on A. Adams Ian. My BJPAs only go back to 1930 but there is an entry here for both an Automatic Changing Magazine and a Bag Changing Box of which the latter is quite similar to that used on the Soho. The Changing Box that I have has a switch on the side which can be set to film or plate. This has the effect of widening or narrowing the slit through which the exposed film is fed back through into the box to the back of the stack.
I do have a load of old MPP film holders that have removable 5x4 film inserts so I may try half a dozen of these to see how I get on.