I have two 12x20 film holders of unusual design. Instead of having the usual hinged flaps, the bottom is one piece, attached to a septum which appears to serve as a guide. There is a precision made locking "tab" attached to each side of the bottom. Each tab fits snugly into a recess in the main body of the holder and is designed to swing out when a small brass plate is moved. Releasing the tabs allows one to pull out the bottom and its septum, giving access to the film guides. When the film is loaded, the bottom slides neatly back into the holder, the tabs are swung back into their recesses and the brass plate is slid into locking position. Very slick. Can anyone shed light on the possible origins of these holders and the effectiveness of the design?