Conn (1917)

I think this is a New Wonder model "French Horn" Alto (later known as model 6E) made by C.G. Conn in 1917. It looks very similar to the New Invention model Mellophone (model 4E), but is slightly larger.

When they introduced this instrument in 1911, Conn described it as follows:

"A splendid substitute to the French Horn in band as to quality of tone. This instrument should be adopted by every concert band and orchestra that does not use French Horns. It has an excellent intonation, a real French Horn quality of tone, and is louder and more resonant than any other French Horn Model Alto on the market."

The 1914 model was built in Eb and sold with an extra crook and slide for C, but by 1917 the model was only built in Eb without crooks or extra slides.

It is a strange horn. It does neither take an Alto Horn mouthpiece, nor a French Horn mouthpiece. Best fit is a Cornet mouthpiece, but with that, the lower register is very difficult. It seems that this is neither a French Horn nor an Alto, but actually a Mellophone with a slightly larger bell than usual.
So, I have gotten a Mellophone mouthpiece and with that the low and middle register sounds good, but higher register (above staff) does not...
It plays in modern pitch Eb, so it is safe to assume it is indeed a Low Pitch model, later designated as model 6E by Conn. I have no idea where to put my left hand while playing so I am still experimenting with that.