T partially dismantled one of the Mentor 13x18 plate holders I received so I could document them.
They are pretty complicated, and have very fine workmanship - they are quite intricately put together. One sign of the care taken in it's assembly is the screws - all the slots on the heads are aligned perfectly. They consist of 4 outer rails made of wood. The upper and lower rails are the same profile, but the two end rails use 2 different profiles (I managed to measure them and I'm drawing up profiles for a later post). There are also a pair of sprung felt light seals behind the RH outer end plate, which stop light getting in once the dark slide has been removed.
The wooden rails fit together using box joints (the metal plates don't appear to do much in the way of strengthening). A thin channel is routed down the centre of each rail which captures a 0.02in steel plate, which forms the separating wall between the front and back plate holders. The overall outer dimensions (including the retaining lips but excluding the dark slide) are 5.7in x 8.6875in (8 11/16ths). The retaining lips on the top and bottom edges are 0.1in wide, and on the bottom edge it is chamfered away in the centre 1.45in from each side.
The dark slide is 0.05in thick aluminium with a brushed outer surface and black rear side. It is 4.96in x 8.125in (measured to the edge of the stamped indent of the handle).
Three sizes of small countersunk brass woodscrews are used in the construction. The largest are used to hold on the outer right-hand aluminium plate. The middle size is used to hold on the corner plates and the smallest are used to hold on the inner glass plate retaining plates. The metal plates are all brushed aluminium of varying thicknesses.
Sorry about the mixed imperial and metric measurements, but it does seem to be made using both systems of measurement (which is odd, as I wouldn't have figured on Germans using imperial measurements).
More to come soon....