I had 2 new 400's in my prestaging area. I could not physically install a particular DTM in the upper MBM in one of the chassis, and had to exert more than normal force to instert in the bottom MBM slot.
I took that same DTM and had no trouble inserting it into any MBM slot in my other 400 chassis. I then retried with another new DTM module in the 1st chassis, and had no problem installing it in any slot. Had no problem installing it in any slot of the 2nd chassis.
Both units are now in service. I am praying a lot that they don't develop the same issues as the ones I referenced a couple of posts back. This is not an official problem yet, but some people that I have spoken with say that there are a small number (and growing I would conclude) of units with backplane issues. Maybe this is the leading edge of a new Q.C. issue?