This screen is made to capture debris that would otherwise get into the vapor cap. It fits just above the lower ring on the cap and is slightly oversized to wedge itself into position. It fits both types of caps.
Each order comes with 2 screens to a package (one for practice) and a 'leash' that is used for installation. Instructions are on the FAQ page of this website.
The screen comes pre-installed on all caps ordered after 7/11/16 and is only needed on earlier caps.