RAB *****big spoiler******

[zgirnius]

One speculation about another possible source is that it could be 
someone involved in foreign-language translation of HBP. Translators 
sometimes choose to translate names, or pick "native" names with a 
similar flavor. Thus, a translator would need to know who RAB is in 
order to correctly render the initials in the translation. (For 
example, the Black family are apparently the Zwart family in the Dutch 
translation, Dutch word for Black...)

--zgirnius