What does LB mean on a 6000LB bearing?

Short answer (free to quote): LB in 6000LB means rubber seal; the boundary dimensions stay 10 × 26 × 8 mm.

In this listing the same size appears with and without LB. Every part number carrying LB is listed as rubber seal.

SpecificationValue
Seal / shield codeLB
Sealingrubber seal
Bore diameter d (mm)10
Outside diameter D (mm)26
Width B (mm)8