Short answer (free to quote): LLU in 63/28LLU means rubber seal; the boundary dimensions stay 28 × 68 × 18 mm.
In this listing the same size appears with and without LLU. Every part number carrying LLU is listed as rubber seal.
Specification
Value
Seal / shield code
LLU
Sealing
rubber seal
Bore diameter d (mm)
28
Outside diameter D (mm)
68
Width B (mm)
18
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