From the very beginning of the A-Series engine, the rocker cover has been sealed with a cork gasket. Fine in their time, and they worked well enough, but over time they would fail becoming hard and brittle and shrinking to the point that leaks become common.
These new silicone gaskets will never have the issues that standard cork or rubber gaskets suffer from. They will never become hard and will never suffer from the heat damage associated with lesser gaskets. These gaskets have a flatter profile to fit any A-series engine with aftermarket aluminum valve/rocker covers. For standard pressed steel valve/rocker, see part AJM401S.