You are correct, it's 3-1/2 by 2-1/4, not 3-1/4, I just looked at some of the old plates I have with the camera.

I can't tell from the German site which would fit the Maximar A. Pacific Rim Camera has some on ebay.com which are Kodak holders, and I do know the Kodak Recomar holders will fit, so maybe I will buy from them. $15 for 3 and $15 for 2 -- more expensive than the Germans but I just don't know what I am looking at there.