Quick follow up. Just did a rough measurement here. There is indeed a difference in the depth (1.5mm) of the recess at the back of the holders. It seems that the theory about the ridge at the left side of the 2x3 Crown's graflok back may be accurate. It looks like the solution is to either grind down the ridge on the camera or grind down the recess on the holder. Can anyone confirm? See pictures below.

horseman vs graflex.png crown graphic ridge.png