Shimano #RD10300 Drag Washer, Key.  The “key” drag washer is used in conjunction with the “eared” drag washer and some type of fiber drag washer to create friction surfaces that “break” the line as it comes off of the reel spool.  Please check the schematics for your reel for the correct part number.  If you need schematics, we suggest www.reelschematics.com.  We are also here to help, just send us an email!