The identification scheme is interesting. It is an attempt to bring the unwieldy, multi-dimensional data of genealogy into to linear form. The scheme starts with Gebhardt Dietrich Gauss, the mathematician's father. I surmise that Dunnington did this in order to include some of Gauss' brother's descendants. From there, it is generated by adding letters a, b, c, etc. to the parent's id string, the letter corresponding to the child's place in the birth order. I have extending this scheme to people not in the book itself. It has been handy in ordering the Gauss Index.