This is a Yankee Hill Machine Co Dust Cover chambered in 556NATO, model YHM-111-556.
YHM caliber marked dust covers for those who may end up changing the caliber of their top end/upper receiver. Assemblies Include the following: Caliber Marked Dust Cover" Rod and Retaining Clip" Spring.
Features:
Caliber Marked Dust Cover
Rod and Retaining Clip
Spring