This cast-bronze relief mold is thought to have been used for making repoussé images. There are slightly variant repoussé images of Kannon at Tōshōdai-ji Temple that form the same mudra and hold the same attribute. The representation of the round face, the torso, and the softly textured draping of the robe follow the style of the early Nara period (710–794). 
Audio guide 

This figure, which has been formed from cast bronze, represents Kannon bosatsu (Skt. Avalokiteśvara). It appears to have been a convex mold for creating repoussé images. This Kannon statue has a single large-scale transformed Buddha (kebutsu) atop its head and exhibits the pronounced realism of the Tang Chinese style. A repoussé Buddhist statue is produced by setting a malleable bronze plate over this type of convex mold and hammering out the form with a mallet.
 
Yasuo Inamoto 
Masterworks from the Nara Buddhist Sculpture Hall at the Nara National Museum. Nara National Museum, 2012, p.125, no.170.