Welcome aboard flip flop gardening. When you take off the disc balance ( hand) wheel) you will find this lobed washer under the stop motion ( or clutch) knob. make sure you keep track of which way it goes one. the way the two center prongs hit the main shaft. If you get it upside down , it can drive you crazy. example below
https://www.quiltingboard.com/members/leonf-u169597-albums19738-576744.html