Shimano rear derailleur pulley assembly Y5X998150 is the OEM updated pulleys for the Ultegra 6700-A rear derailleur and are the direct aftermarket replacements for the following derailleurs and feature a sealed bearing tension pulley:
These are original to my M591 rear derailleur and they work well with it when the old one's needed replacing after 3000 miles. They last plenty, I might have gotten away with simply servicing the old ones but their teeth were noticeably worn enough to justify replacement, and the replacements are smooth and shift better.