A large photo of this mask is shown on page 36 of Masks of the World by Ibold and Yohn. The caption reads: Nepal monkey mask – Terai region, Nepal and South Himalayas, 14 inches, painted wood. Represents a monkey or possibly another animal. It is an excellent carving with great aesthetic appeal and rather thin walls. Folk art carvings from the Terai are seldom sculpted this gracefully.
I believe there is a treacherous monkey character somewhere in the Ramayama, that very long, ancient epic poem written in the 11th century. The old, used masks of the Himalayas are always interesting. Unfortunately, we still see a great deal of tourist junk coming out of the area.