One Viton pump head center plug o-ring seal is included with the socket. 

Every once in a while we get a customer with a pump leak that is unlike most leaks. There is a center plug between the four injector lines that has an o-ring seal. This socket is required to remove and replace the center plug. We offer this sockets rentals since repair is uncommon. 

Installation of center plug seal: Remove injector lines from engine. Remove center plug with our 24mm socket. Remove the old o-ring seal from center plug, clean the plug and o-ring groove thoroughly and then install the new o-ring seal included with this package. (Do not loosen the 12mm-headed calibration screw in the center of the center plug.) Replace center plug and injector lines, re-prime pump and start the engine. Check for leaks. There are no special tricks or anything to fall out of place when doing this procedure. It's a simple replacement of an o-ring and nothing more. "Good and tight" (Gutenteit) is the official Bosch torque spec ;)