The Intel Xeon E5-2600 is a large chip and can have 8-12 DIMM slots per CPU. In dual socket configurations, that means there is not a lot of real estate for heatsink assemblies. The standard square ILM is fairly large so there is an alternative, the narrow ILM. The big advantage of the narrow ILM is that it allows for a more dense layout. Just to illustrate the difference, here is a picture of both options.
The motherboard on the left (a Supermicro X9DR7-LN4F) has the narrow ILM. The motherboard to the right an ASUS Z8PA-D8) has the traditional square ILM. One can clearly see the size difference between the two mounting options.
When it comes to heatsink options, there are a few narrow ILM options available. On hand we had two different types from Supermicro. One is the Supermicro SNK-P0048PS (about $32), a passive 2U cooler, and the Supermicro SNK-P0050AP4 (about $40) an active 4U cooler. The two heatsinks are LGA 2011 only and meant to cool Intel Xeon E5 series processors in rackmount cases. The active Supermicro SNK-P0050AP4 includes a 4-pin fan, so a pair can cool an idle Intel Xeon E5 server and be barely audible.
One may notice that the Supermicro SNK-P0048PS is a sizable piece of aluminum. The narrow ILM is built into the base. Conversely, the 4U tower cooler Supermicro SNK-P0050AP4 uses a removable mounting clip design. 4 screws secure the Supermicro SNK-P0050AP4 mounting bracket in place and it comes with both the square ILM and narrow ILM versions. That allows us to look at the square ILM v. narrow ILM on the same heatsink/ fan assembly.
As one can see with the two Supermicro SNK-P0050AP4 coolers, the narrow ILM saves a good amount of room on the side of the LGA 2011 socket. One major note is that the square ILM has many more options available. The reason here is that many consumer heatsinks support the LGA2011 square ILM. The narrow ILM has significantly fewer choices. Still, one needs to take care to ensure to match the motherboard ILM with the heatsink for LGA 2011 platforms.