This is a meliphone, in the same key as a French horn. It has three valves like a trumpet, and no trigger. It is heavy like a double horn, but without the fourth valve, it cannot be classified as such. The meliphone sounds almost exactly like a French horn. It plays in the key of F.
Old fashioned french horn (single high Bb ?)
from en.wikipedia: Public domain image from Websters Dictionary 1911.