Armitage Shanks / Ideal Standard Univalve toilet cistern fill / float valve diaphragm washer.
Suits the newer twist cap style of inlet valve / ball-cock supplied with many Armitage and Ideal standard toilet cisterns. It won't fit the older "Hushflow" screw top valve.
May have white or red plastic centre nipple (apparently this is a colour coding system used in the factory to identify different batches).
Please study all picture(s) very carefully to make sure you are buying the correct washer.
When these washers are new they have a smooth surface. After they've been used for a while they develop 18 little ridges pressed into the rubber.