This is the New Valve Seal used in this guide, available at hardware stores and home centers.

Using this New Seal installation shortcut method, it will NOT be necessary to do the following (of the original instructions that accompany the New Seal product): "Turn off water supply to toilet and drain tank" and "Unhook trip lever..." [any "chain" will need to be momentarily unhooked.] and "Unthread stop cap..." and "Lift float assembly off....."