Idle Gear Retainer #BNT0686 or #RD 2404, Shimano.  This retaining “c” clip is used in many of Shimano’s fishing reels.  Just be sure that you check the schematics for your reel for the correct part!  Don’t have schematics?  We recommend reelschematics.com.  they have fishing reel schematics for most reels manufactured in the last 40 years!  Still can’t fine what you need? Send us an email and we will see what we can do!