Up for sale is a new Volvo CH62584 Cap.  From my research online, I see the terms: "grease retainer".    

I have 2 of them for sale, as shown in the actual photos. 

One purchase will get you one CAP.

I also found this information in my search...

CH62584 Cap Volvo. Heavy, G700 MODELS, G700B MODELS, G900 MODELS, G900B, Cap