YHM has long been a supplier of Caliber Marked upper receivers, but they soon asked themsleves ""What if we want to change the caliber of that upper receiver?"" For this reason, they now offer caliber marked dust covers for those who may end up changing the caliber of their top end/upper receiver. SPECIFICATIONS Assemblies Include the following: Caliber Marked Dust Cover Rod and Retaining Clip Spring Mfg: Yankee Hill Machine Co., Inc.