The sharp beak-like nose indicates that this mask represents a type of mountain demon (tengu).
Inscription: Inscribed under the chin: Shōtoku san, midzunoto-mi, sai ni-gwatsu kisshin Miōchin Ki Muneaki giōnen sanjuichi (no) koyomi (or reki?) Bukō, Kanda, ni oite kore wo tsukuru.
(Translation: Miōhcin Ki Muneaki, 31 years old (by the calendar?), made this in (the district of) Kanda, in Bukō (i.e., in Yedo in Musashi province; modern Tokyo), on an auspicious day in February, 1713, Year of the Snake (and the thirtieth year of the 60-year cycle).)
Grancsay, Stephen V. "The Japanese Armor Gallery in the Metropolitan Museum of Art." The Connoisseur 148 (September 1961).
Boger, Batterson H. The Traditional Arts of Japan: A Complete Illustrated Guide. Garden City, N.Y.: Doubleday, 1964.