Congrats to him.
If he found it at Milledgeville, those anvil caps are toughies because in my experience, having owned several, they take a slightly larger than usual diameter anvil cap. A regular CS one sits down in there, but it is not a tight fit.
I do not have any spare CS anvil caps in either size at present, but I do have some of the sliders that go in those shells. I have some US anvil caps which look pretty much the same from the outside, but I'd not advise mixing one of those into this.