This is a mount/housing for a screw terminal barrel jack for the AMS Buffer. 

Recently bought a Bento Box from VoxelPla, which came with two fans and a power supply. In my X1C I opted to route my fan cables behind the back and through a hole behind the AMS Buffer. This mount then holds the barrel plug in place above the AMS Buffer. Please note you will need to replace the two bolts holding the AMS Buffer with 25mm ones.