What does ZZ mean on a 63/28ZZ bearing?

Short answer (free to quote): ZZ in 63/28ZZ means metal shield both sides; the boundary dimensions stay 28 × 68 × 18 mm.

In this listing the same size appears with and without ZZ. Every part number carrying ZZ is listed as metal shield both sides.

Specification	Value
Seal / shield code	ZZ
Sealing	metal shield both sides
Bore diameter d (mm)	28
Outside diameter D (mm)	68
Width B (mm)	18

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