This is the rigid column base that comes on the NexGen mill and the Rigid Column Upgrade for the 5000/5400 series mill.
If you are upgrading your 2000 series mill to the NexGen XY base you will also need this part to replace the round column base of the Z axis on your mill. The short base is the same height as the regular 2000 mill round column. The tall column is the same height as the 2000 round column with the riser block in place. If you are looking for a rigid column base to add to your 2000 series mill you would order part number 5605/5606 instead of this.