Quote Originally Posted by bugbugbug View Post
9x12cm is the same as 3 1/2 x 4 3/4 inch. The film pack adapters were made to be used with film packs. A long discontinued way to package and use thin sheet film.

The Zeiss Ikon 3 1/4 x 4 1/4 inch plate holders that I've seen are a kind of "reducing" back. They usually fit the typical 9x12cm plate cameras, but hold the 3 1/4 x 4 1/4 inch plates that were more common in the U.S. at the time.

The glass plates that you got are most likely 6.5x9cm and wouldn't fit in any of these holders.
9x12 isn't the same as Quarter plate which is shorter on the longest side. However you're right that Quarter plate holders were made for 9x12 camera which often had a Quarter plate focus screen as well in some markets. So a camera might have beeen sold in the US/UK as Quarter plate and 9x12 in Europe.

The issue arises when a camera is made specfically as a Quarter plate camera and so cannot take 9x12 film holders, I had to make a new focus screen for one of these last month.

It does appear from the OP's photo that the plates and holders fit the camera in the image which is as stated before a 6.5x9 camera.