Picked up out of curiosity at yesterday's flea market.
There is no identification except 'Made in England' and 'R8' stamped on the holder. It is made to hold 12 5x4 glass plates or 12 5x4 sheet films in idividual holders numbered 1 to 12. The numbers can be seen through the ruby window on the magazine. The holders supplied with this example were for plates but a switch on the magazine can be set for plates or film so I assume film holders must have been available too.
Pulling the tambour slatted cover across the holder does the equivalent job of pulling the darkslide in making the plate ready for exposure. Once exposed, pushing the cover back ejects the exposed plate into the green suede pouch at the end of the magazine. The next plate is then automatically placed in readiness for the next exposure when the whole process will be repeated.
The edges of the holder are a bit like the fast loading design used on Thornton Pickard plate holders in that the flange along the edge of the holder is only partial, allowing for faster attachment and removal.
I can only guess that this is most likely from a Marion, Houghton or Thornton Pickard camera being English made. It is probably later in that there is an option for film or plate and the fact that it is for 5x4 rather than half plate or quarter plate.
An ingenious and nicely made item but a mystery.