Any ideas as to where she came from? Can you ask the previous owner as to what some of the possibilities might be?
Again, you can never know where a horse from, best bet is to go from owner to owner of the past and you will have a rough idea, you never know then either so don't take it to heart cause people fib when they want things to look REALLY good.
unfortunately I can't ask horse's previous owners about his past, breed etc, because it was bought by a foundation after his accident, from some horsedealer. This brand is my only track.
Then the most I can suggest is if you are really determined ask the Jockey Club, AQHA, if they have a registry to appendix's try that as well, see if they keep records of brands. If not I wouldn't worry about it, just sit back, relax, cause it looks like you have a good horse there.