Top quality, made by MBD. This cap sits on top of the sprocket spring, slides down the sleeve and is held in place by the drive sprocket bolt. 
 
These always have been a pain to fit and many damage the castellations trying to fit them, and if this is not seated correctly on the sprocket sleeve when you put your crankcase side on and tighten up, the bolt will crack or even come through the chaincase side! MB have made the castellations with slight tapers to help fitting.