Images of this type are extremely rare. Ingholt illustrates a fragment where the Bodhisattva lacks any attributes, identifying it as a possible image of Siddhartha addorsed to his enlightened self as Buddha. Here the identification as Maitreya is given by the bottle and hairstyle, thus combining the current and future Buddha.
Another closely related example is in the Russek Collection, Switzerland, see R. Russek, Buddha zwischen Ost und West, Skulpturen aus Gandhara/Pakistan, 1986, cat. no. 60, where the Bodhisattva holds a lotus flower and is identified as Avalokiteshvara.