SWK40 NB:Made in Japan:NB Linear Systems Square Flange Type Nippon Bearing Linear Systems SWK40 NB 2 1/2" Bore linear motion ball bushings/bearings, the inner diameter(bore) is 2 1/2" 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.