We know Undertaker has 7 lockets total. Claudia P. is Claudia Phantomhive, that much is certain, but there are still 6 other people. I went back to see and yes, there are at least 3 other recognizable initials (Source).
Crack theory of the day: the C, B, and G aren’t just unknown names. There are characters already in the story with those surnames.
Another G could be Greenhill, but judging from the way UT treated Herman, I doubt it. But we’ve yet to see UT interact with John Brown and Charles Grey, maybe with good reason? They both happen to be the Queen’s trusted servants and Brown at least, is suspected of not being quite-human. There’s also Grey’s intense dislike for the Phantomhives. And we’ve already seen how “taken” UT was with Aleister lol.
So there you have it; crack theory of the day- Druitt, Brown, and Grey are also connected to Undertaker.