I shot a 12X20 Korona for a while. It had Korona holders and I checked the T dimension between a couple of holders and the dimension was not consistant. So what I observed and what I have heard over the last several years is that the world of ULF back when the Koronas were made is that there was no standard.

My Korona holders did not have a means to hold the film at either end other then the light trap on each end. There was certainly some film slop inside the holder. I had the same questions and concerns about film flatness when I started using the camera and others indicated to me that it really wasn't an issue for them, in their experience so long as the camera was not pointed downward.