Looks to me like your chain has spent a lot of time running loose? Judging by the build up of chain lube in there I don't think the cover has been off in a while and it kind of hints at the fact that the previous owners chain maintenance regime wasn't that great? All that build up just attracts dirt and grit and ends up like grinding paste and continually rubs on the chain.

Maybe that the chain is goosed? What is the condition of the sprockets like? How much lateral and longitudinal movement is there in the chain? Worn cush drive rubbers and/or a sloppy sprocket carrier bearing can cause premature chain wear and excess lateral movement. What are they like too?

__________________