Clutch master cylinder rebuild kit for master cylinders manufactured after 1985. The early clutch master cylinders used a cup type seal which was later replaced with two ring type seals. This rebuild kit includes everything you will need to rebuild your late style master cylinder. 
 
Original AP post 1985 master cylinders featured a groove just near the base (see image). Note that the later reproduction masters do not have this groove. If you have any doubts, use GRK3007 as it can do both.