Madame Bone's full name is Amelia Susan Bones. Her niece Susan was named after her aunt's middle name. So her initials are AMB.
Darn. Well it was a good theory while it lasted. LOL
Ah nuts. I did think that there would be more about her death somewhere in the book, and she was the only AB I could think of yesterday. Today, when the topic of Regulus came up, it seemed so obvious.
I have a feeling that JKR is sitting in her castle, reading this and many other threads and laughing hysterically.