The block retainer sits on the back of the G10 airless spray gun body. It is held in place by a small nut (9910201). The G10 spray gun was a common airless spray gun that was included with a variety of airless paint sprayers, including the SW317/419/423, ED2005/2205, and many Amspray models.
Part #: 0278361
Part #: 0149700