SPX Stone/Fenner OEM reservoir tank o-ring. It is universal and used on virtually all Stone/Fenner hydraulic power units since the mid 90’s. It is designed to slip over the lip of the endhead/manifold where the reservoir flange bolts down. If you are replacing your reservoir, it is also a good idea to replace this o-ring as it can get damaged or worn over time. The o-ring is nitrile.