Heres what I did years ago to my former Toro..FR to FX Air Box mod https://www.lawnmowerforum.com/showthread.php/52096-Kawaaski-FR651V-FX-air-box-conversion
Weird that the nut is still on, think that would have prevented it from falling out or off..
Definite wear pattern around the 1-3/8" point from bolt head.