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.
Originally Posted by bugbugbug
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.