It's part of the XTR crank spider tool, the other piece is the 6 or 8? notch socket thing. Forget the part number but it's the same notched tool as the XTR bottom bracket.

It's used with the notched piece and a hollowtech crank bolt to hold the spider tool onto the crank so it won't slip when removing the lockring that holds the spider on.

Edit: IIRC it's actually double sided to work with standard crank bolts also. All those late 8speed and early 9speed Shimano cranks had the splined crank spiders held on with a lockring.