SWK48 NB:Made in Japan:NB Linear Systems Square Flange Type Nippon Bearing Linear Systems SWK48 NB 3" Bore linear motion ball bushings/bearings, the inner diameter(bore) is 3" inch dimensions with steel retainer, bearing is a high accuracy grade linear bushings, series mainly used in the USA, NB brand (Nippon Bearing Linear Systems), made in Japan.