Mystery mask
Luba - Songye - ?
Click on any image to see high resolution version
My initial conclusion was that the mask was of Luba or Songye origin, most likely Luba, and was a form of a zoomorphic mask produced by them.

The book that was of most interest and help to me was the book 'Beauty and the Beasts" by Marc Leo Felix that was written about Kifwebe and
animal masks of the Songye, Luba and related peoples. My mask shares stylistic similarities, especially the pronounced forehead, to masks on
page 97 (ill. 144, an animal mask) and on page 98 (ill. 151, a Lomotwa mask depicting a woman). In the text in this part of the book it describes
how masks depicting women were often danced along with animal masks.

The mask is made of the same wood that I've seen many Luba and Songye Kifwebe masks made from and has the attached bark fiber raffia that is
commonly found on masks from these cultures as well.

Directly below I have placed scans of the 2 masks I mention from the book that had features that resembled my mask.
ill. 144 - Animal mask - no attribution given but I believe it may
have been Lomotwa as well.
A member (Walberto) of the African Arts and Culture Discussion Group added the following:
"My guess is that they are inspired by the facial markings of
certain species of monkeys endemic to Luba country.
Some varieties of red colobus and Diana monkeys sport
circular, light colored skin around their eyes and
black patches across their mid-face. As wildlife is
a frequent theme in African art and in masking in
particular, a good bird book such as Princeton's
excellent handbook to the birds of West Africa and The
Kingdon Field Guide to Africa Mammals are, to my way
of thinking, are invaluable additions to any serious
African art library."

Below are photos of Diana monkeys. There is a very strong resemblance in the face of the monkey and the face of the mask.