DG/LS/AS (Alexander Schleicher) control cable pulley with ballbearing 50mm.
Commonly used for the towhook cable in DG/LS/AS gliders.
Also found in some other types of gliders.

Specs:
Diameter: Ø50mm
Bearing mounting hole: Ø6mm
Suited for cables: Ø2.4mm and Ø3.2mm (Ø2 - 3.5mm)