Its at the end of the stem. There should be a washer on this end piece that tightens up onto a concave removable part. When the washer on the male goes south the metal to metal scouring on this seat scratches it and a new washer will never fix the problem, the seat has to be replaced.
There is a special tool to remove this seat and then because of a gillion sizes and shapes, you have to take this seat in and have an identical one purchased and installed. The tool costs about $4 and the seat is about $2.