Maybe the outer part is pressed on and this one isn't supposed to be dismantled & serviced at least without at least the outer item being replaced ? Perhaps sealed and disposable for the life of the sealed unit ? What identifying items are there for a possible model identification ? This is the best I could find, but also it has flat edges for using a wrench.

http://www.parktool.com/blog/repair-...and-adjustment

HUB-CARTRIDGE BEARING TYPE

These types of hubs use an industrial type-bearing unit. The inner and outer rotating races and ball bearings are installed as a unit. These hubs are not serviceable in the sense they are overhauled and adjusted. When the cartridge bearings are worn, rough, or have play, the cartridge should be replaced. The bearing unit is then removed and a new one pressed in. This requires special tools and is best left to professional mechanics.