This seal and retaining ring combination is used to cover the weep hole on combination valves used on multiple 1970s and 1980s Fords. The seal is made of 70 durometer EPDM rubber. The retaining ring is made of black phosphate 1060-1090 spring steel. They're designed to replace the small rubber plug that seals the weep hole at the end of the proportioning valve nut as shown in the picture below.
The listed price is for the combination of one seal and one retaining ring.