Hi Brenda. I don't think there's really another document that would constitute "proof." Usually a birth certificate rules for establishing the name at birth. It sounds as if your dilemma is only about given names(s), not LNAB, so that's not too serious a problem. If you find records that seem to indicate g-gf began using a different given name at some point in his life, you can state that in the Bio, and it's pretty much your call as to how to show first name, middle name, nicknames, and preferred name. If you are in the U.S. one type of record that I find to be very useful for determining how the person identified himself are the WWI and WWII Draft Registration Cards. They were typically completed by the subject himself, and they seem to have real names and dates as the subject knew them.
