This is a natural (valveless) horn commonly called a Cor de Chasse or Hunting Horn.  Horns of this design have been used musically since the 17th century. The Cor de Chasse produced about 12 tones of the natural harmonic series with the most useful range for melodic writing in the upper harmonics (the "clarino" range) where the natural harmonics are close together.  This horn was made by J. Milllliens, A.R. Crespin, Dugast. Paris.  and designed for modern use in cor de chasse ensembles. It is pitched in Eb.  Similar instruments were involved in the Brandenburg Concerto #1, and other repertoire for baroque horn (pre-hand horn technique).  See bottom photo for a demo by collector extraordinaire Dr Hank Merideth on proper technique for playing a Cor De Chasse.