This interesting device is a Linear Bearing Platform. These are designed to simplify mounting hardware to a linear bearing. The platform is comprised of a standard LM8UU linear bearing inside of a larger aluminum block. The bearing is held in place with a few snap rings and can be replaced. The block has four tapped holes to attach the motion platform or other hardware. Typically, these platforms would be used in pairs, mounted on opposite rails to allow the movement of much larger platforms and gantries.

This specific Linear Bearing Platform is designed for 8mm rod.