This is a double ball sleeve. The ball sleeve consists of a polymer cage with hardened steel segments to guide the balls within the system.
It is also called a ball bushing or shaft guiding system because it allows for extremely precise linear movement on circular shafts.
The recirculating ball system allows unlimited travel with a low coefficient of friction and quiet movement. The item you will receive may differ in some features from the one shown in the photo, depending on the production batch. Despite this, the functionality is maintained.