I have a 9x12 Bergheil that came with four plate holders. There are some slight cosmetic differences in the backs but they all fit. As already stated the basic dimensions are 100 x 150 with a 7mm locating lip. The locating lip measurements are indicated by the blue lines I added to the photos. One of the holders came with a reducing insert to hold 6.5x9cm plates. That will come in handy as I have plenty of plates in that size. I don't have any 9x12 film inserts however & I'd really like to get some as I have film that I can't shoot because of this. I've nothing to trade but I do have money.
I've also included a detail of the top corner of the camera back to show the cutaway that the locating lip has to fit with. Best advice I read when first researching plate cameras was to buy one that comes with holders that fit the camera. Unless you already have a good collection of holders it can be difficult to get the right ones. Thanks to Ole for that tip.