Here is an 1863 IHC that has what I'm calling a CUD. As you can see from the pictures, I'm not entirely certain this is the proper attribution? It has all the characteristics of a CUD, but the dentrils are still intact? Shouldn't the area in question be void of details? At least that's my understanding of the term. I know I can count on the group for some insight.
Great example of a Retained Cud. The Rev. Die broke at that area, but the piece did not leave the die, due to it being the reverse die. It was 'retained'. If it fell off the die, you'd have a full Cud in that area. You can see the die displacement at the top of the retained area.
