The International standard for film/plate holders is post WWII 194/7 I think, and some new cameras still took non standard book form holders well into the late 1950's. Watson and Gandofi were two companies still making book form holders
I must have at least 15 or 16 MPP holders now, 6 are plate holders 2 with the same film inserts, others are identical except MPP riveted the inserts in place from new. Some MPP holders have a different register as did some very early MPP cameras. I have checked mine and they are all standard. MPP holders do have slightly wider edges holding the film so the film area is a touch smaller. They are very well made and despite high prices on Ebay from certain sellers can be picked up quite cheaply. (£2/$3.15 is about the average I've paid for them).
Originally Posted by paul_c5x4