Base, Solid Column, HiTorque Mini Mill 

In your instruction manual, this part appears on the parts list as:

Item No	Models
1	LittleMachineShop.com 3960 HiTorque Mini Mill LittleMachineShop.com 3990 HiTorque Mini Mill LittleMachineShop.com 4190 HiTorque Mini Mill Various SX2LF-US SIEG Mini Mill LittleMachineShop.com 4962 SIEG Mini Mill (X2D)
156	LittleMachineShop.com 6450 HiTorque Mini Mill

Click your model to see the complete parts list.