hi ,yep you got it it's a 520 ''x''or''o''ring.

mine had a regina x ring from new which wore out and had tight spots even though a scott oiler was used.though on one occasion the bike was washed with an very high pressure hose which may have forced water passed the x ring.the chain was really shagged at 5500mls.

im sure you will check yours thoroughly, no point risking a breakdown

bye