Hi Peter,
In the position that you have it pictured, the cover that removes to allow you to remove the plates is itself covered by the roller blind. If you turn the box over you should be able to see inside it. If there are plate holders in there you will see either a flat black metal surface or an opaque glass (if a plate is loaded) or flat film surface (if film is loaded. If you apply pressure to this surface it will depress and spring back when you release it.
The roller blind on the surface we can see in the picture should pull back across the face of the back revealing a cover with a ruby window in. My guess if it ijammed is that a plate is partially stuck in the exit hole that is used to eject it into the suede bag. You may be able to feel this and push it back.
Either way, what do you want for the holder in it's current state? I may be up for a gamble!!
Steve