Hello Ole -
Although beyond the scope of your original post, I have a question on european style film holder that I hope you could share some light on.
In a weak moment I bought an Ica 5x7 Universal Jewul in what appears to be very nice condition. It has a spring back, not as a standard US camera does, but it isn't a slide on back like my 10x15 Ica Maximar either. The spring back on the Jewul appears only to open enough for a very thin holder. I took the ground glass off and it appears that the holder must have outside dimensions of about 157 x 208 mm but that the holder must have a rebate to fit over a thin ridge that measures 131 x 181 with an actual film or plate window of about 129 x 170.
I'm not sure my description is clear enough to make sense but does this sound like any 13 x 18 holder you have heard of?
Thank you for any information.