its rare to have them on the back legs, only breeds i know who have them are the big sheep guarding breeds( kangal,swiss mountain dogs etc.........) hope you find her a home looks a nice young bitch.
i bred a litter of the same cross 6yrs ago pups were born with due claws
front/back the first time id seen this as well was 2 genuine firstxs bull/ghdxwheaten/ghd