Ngombe stool / backrest often called "tip stools" - Democratic Republic of the Congo
Each on a single peg leg, each openwork seat incised all over with parallel lines in geometric patterns, brass tacks as ornamentation on edges (half
missing on one). DR Congo. One:  22 3/4 x 9 x 8 1/2 inches. Provenance:  the estate of Jean-Pierre Hallet, Malibu.

I've seen these referred to as Kuba stools and Ngombe stools / backrests.
EXAMPLES BELOW ARE NOT IN MY COLLECTION - THEY ARE FOR REFERENCE PURPOSES ONLY
A pair of Ngombe backrests - Sotheby's Nov 15, 2002 - Lot 74

Property of a Gentleman (That just makes me laugh)
TWO FINE NGOMBE BACKRESTS

Estimate 2,500—3,500 USD
Lot Sold.  Hammer Price with Buyer's Premium:   2,151 USD
The Hamill Gallery in Boston has a few nice examples of these stools / backrests.
CLICK HERE to go to the page on their website to see their offerings.

They have them listed as Kuba stools
Photo above was taken by Michael Auliso of Tribalmania.com

The photo was taken at the 2007 San Francisco Tribal and Textile Arts show. The space in the
photograph was the booth of B.C. Dentan of San Francisco. On the left hand side of the photo
you'll see a Ngombe stool/backrest. It was a beautiful object, decorated with lots of brass tacks.
CLICK HERE to see the Ngombe chair in my collection (shown above)