It does seem like the Combi boxes have a different way to attach the drawer section: the top and bottom parts reach halfway down the sides of the drawer. I'll tinker when my two Systainer³ Sortainer/3 M 337 arrive.
See my photo above. Your description of the Combi boxes also applies to the Sortainer/3. (Edit: No, you can't see that in my photo because of the overlap of the top and bottom part. But when I look on the outside, the split line is halfway down the sides of the drawer.)
I am unsure about the compatibility of the large drawer compartment in the Sortainer/2. I have not yet checked the split location and the number of splits in that compartment.