Push nut, star washer retainer for inside front caster wheel and inside lower chassis for front axle. This retaining lock washer is used for front caster wheels on 430X, 430XH, 450X, 450XH, 520, 520H, 550, 550H, 550 EPOS, & 550H EPOS model Automwers.
 
Click HERE for parts list for caster wheel assemblies 
Complete caster wheel assembly for 300 series available HERE
Complete caster wheel assembly for 400/500 series available HERE