Quote Originally Posted by grat View Post
Pardon my ignorance, but isn't the critical factor the location of the film plane? The film, and the ground glass, need to be on exactly the same plane for focus to remain consistent when you insert the film holder(s). How thick is the glass on the one that's raised versus the one that isn't? Does it have consistent focus when you take pictures? If so, that's what's relevant.

As for the sloped corners, it would allow precise control over the depth of the ground glass by trimming the corners, if you couldn't get glass at a consistent thickness, and would eliminate the need for any type of shim.
Yes they should match, ideally. You focus to the ground glass and then put the film at that location. If you focus wrong because the ground glass is not aligned (e.g. you put a fresnel between the ground glass and the lens) or because the holders have issues (OP's problem) then you will turn a sharp dot into a circle, which is the issue of depth of focus.