This is a mechanical brake lockout cap that is an "upgrade" option for the model. At 2-1/2" in diameter for the top and fits a 2" hole this cap commonly replaces caps for the A-60, A-75 and A-84 brake actuators. It allows trailers equipped with drum or disc brakes to be backed up over soft ground or up inclines without fully engaging the brakes. The cap is designed to disengage when the trailer is pulled forward after backing up.  It replaces the existing plastic cap, It is zinc plated for superior corrosion resistance. Installation very easy!