Steve, it was MPP plate holders I ws referring to, I bought maybe eight last year with the removable dfilm adapter and I have more which are the same except the film adapter parr is screwed or riveted in place. However that adapter plate is what I called a pressing, I think it'd be too thick to use in your septums. The other one are thin enough,
I did have a brainwave which I'll try and test with some 9x12 plate holders in the next few days, If it works it should work with any format plate holders.
My guess is your changing box is late 1920's or 30's.
My BJPA's are not too bad for the 30's and 3 in the 20's and a couple earlier, extremely useful resources. I've every one from 1939-63 now. But I have many books from the 1880's onwards.
Perhaps the most interesting of the BJPA's are the 20's and 30's. There's the suddend grouping of companies in the 20's in both the UK & Germany and then the quantum shift from LF/Plate cameras to Miniature 120 & 35mm not really mirrored in the same way in the US. This is mirrored in other ways with a shift to utra fine grain and high definition developers in Europe which carries on after WWII with Kodak selling products in Europe never sold in the US. Needs another thread to expand this though