We often times get a lot of questions pertaining to the IBM Rail Kits working in Square Hole 4Post racks. Most times the question relates to the IBM x345, x346, x365 and the x366. The IBM OEM Rail Kits will usually work well in most cases. The confusion comes due to the fact that the IBM Instruction manual is not very clear.
The IBM rail kit mounts using 2 pins that when retracted allow the rail itself to wrap around the outside of the of the rack as illustrated in the picture below.
Once the rail is positioned properly when you release the the pins they will then slide through the square holes on the rack then through the flange on the rail securing the rail to the rack. This is then repeated for all remaining corners of the rack.
If all else fails and this does not work you a can use the third party rail kit that is available for these servers. The third party rail kit will work in most 4Post Racks that have both round holes and square holes.