Chris, 13x18cm bookstyle holders are very common in Europe. Unfortunately they come in about 100 different external sizes, only one of which will fit any given camera. I was fortunate enough to get a 13x18 plate camera with four matching holders. Three of the holders can take 5x7" film inserts, in the fourth one they will slip. Apart from that I can find no difference between them, even all internal measurements are the same!