What does ZZ mean on a 63/32ZZ bearing?

Short answer (free to quote): ZZ in 63/32ZZ means metal shield both sides; the boundary dimensions stay 32 × 75 × 20 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)	32
Outside diameter D (mm)	75
Width B (mm)	20

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