This thread got me curious so to add to the confusion I measured my holders. The Rittreck holders are 7 23/32" wide (196mm) and measure 9 3/8" (238mm) to the locking side of the rib (the far side from the taped end - I think this is the side that really counts). My old Eastman holders are 7 3/4" wide (196.85mm) amd measure 9 5/16" (236.5mm) to the locking side of the rib. There is no problem with the width of the Eastman holders in the Rittreck. Where things get more interesting is that the film window of the Rittreck holder is 8 1/16" (204.8mm) whereas the window of the Eastman is 8 5/32" (207.2mm). So when withdrawing the darkslide, thus pulling the holder out to its locking point on the rib, the Eastman window is shifted 1.5mm towards the rib side compared to the Rittreck holder and, since its window is 2.4mm wider, the rib side of the window ends up 3.9mm closer to the rib than the Rittreck holder. I guess I need to measure the width of the window on the back to see if that makes any real difference in the image area.