This is a Tibetan Buddhist Citipati mask made using the hammering technique in Nepal. In English, it is Citipai, but it is pronounced Chitipati. Chitipati is often depicted in thangkas as dancing male and female skeletons, but it is also depicted as a skull encircling Mahakala's head or as a single skull mask. It is a symbol of the emptiness of life and eternal reincarnation.
The material is brass. Year: Around 1970