SHIMANO ROLLER CLUTCH ASSEMBLY #RD16381
 
This is the one-way bearing assembly to stop the handle winding backwards
 
This one way unit can be found on many of the Shimano fixed spool reels such as;
 
Baitrunner 6000ST

DAIWA ROLLER CLUTCH/INSTANT ANTI-REVERSE BEARING/ONE WAY BEARING G84-5601

When your handle unexpectedly turns backwinds this is the culprit

SHIMANO BAIL SPRING GUIDE # RD2532  A New Old Stock Shimano Bail Spring Guide A
For the older Shimano Bait Runner spinning reels

3X6X2.5 STAINLESS STEEL SHIELDED ECONOMY BALL BEARING

A Stainless Steel 3x6x2.5 shielded economy ball bearing