The SRB series are units manufactured by Unitec company. They have needle roller radial bearing that have solid outer ring plus an extended inner ring.
The unit's side surfaces play as tracks for the two axial cages. These things are centered on the unit's outer ring. The unit is applicable on any operating requirement such as support on screw of various machines having a precision journal.
Further, the bearings were preloaded through the locknut for the user if they want to obtain stiffness.