Special washer with a diameter of 8 mm with a rubber seal . Suitable for models of pumps Koshin 2-3 inches inlet / outlet  STH-50-EX, SEH-80, SEV-50, KTH-50, KTH-80 etc. 
It is used for fastening the Flange bracket to the engine PTO flange with hexagon socket bolts М8 CS (Koshin ID must be checked wit the parts list for exact model). A set of 4 is required for one pump.