SWF64 NB:Made in Japan:NB Linear Systems Round Flange Type Nippon Bearing Linear Systems SWF64 NB 4" Bore linear motion ball bushings/bearings, the inner diameter(bore) is 4" 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.