NBC Thrust Ball Bearings 51XXX series

NBC’s catalog confirms they manufacture “Thrust Ball Bearings” under their ball bearings range. 

These bearings are designed to support axial loads (i.e., loads parallel to the shaft axis) but not primarily radial loads. 

NBC mentions features: composed of a pair of rings with raceways on the faces, balls in-between, used for applications with axial thrust loads.