Twin row ball bearing used on all Single Cylinder models (pre-full width hubs) on the brake hub side of single sided front hubs (although strangely not used on SOHC conical front hubs. They are also used on all rigid\plunger hubs on the brake drum side.
As always, this bearing is a high quality named brand bearing - NSK (i.e. not a chinese bearing!). This twin row bearing was always much more expensive than the single row fitted to the other side, and is now getting quite difficult to find - but as at the end of 2012 I have managed to get another batch!