Heritage say there's a graze on her cheek. Might be that. Baldwin's did list it as the proof, just strange they implied the obverse was less certain to be from a proof die than the reverse, and gave it a low estimate of £3-400. Perhaps the graze, if it was that, put them off. If anything I'd have thought the obverse would be easier to tell proof-wise based on the sharpness of the hair.