Compact symmetrical 4 speed AWD gearbox with differential lock from BrickLink Studio
Leic Power Functions Motor Set 113Pcs MOC Electric 6-Speed Synchronous Gearbox Assembly Kit with M Motor Compatible with Lego Technic : Amazon.co.uk: Toys & Games
Ultra Compact 8 Speed Sequential Gearbox - LEGO Technic, Mindstorms, Model Team and Scale Modeling - Eurobricks Forums