Well if it's a Gebrüder Mönnig, it's a very old one. The company started out in Markneukirchen in 1926, but all the horns I have photos of--including the ones that I haven't yet had a chance to upload--have serial #'s that following the numbering convention shown on my website:
2817 [or insert other serial # here]
M
1938 [likely year of production]
The earliest year of production I have on any of the horns is 1936, and is on a tenor.
However, even that tenor has the low C key shape similar to that of the JK's and key guard "foot" shapes nothing like the ones pictured.
The leaf-shaped ones on the alto currently on eBay resemble those on
early Pierrets. I'm not suggesting that the horn is French, or a Pierret, but I can't remember seeing those type of key guard feet on German horns before. I'd have to pour through my pics to see what I could turn up.
Oh, upon closer inspection. There's something about the eBay horn's engraving that doesn't fit into the Gebrüder Mönnig style as well. It doesn't look like any of them. Not to mention, it is missing the Gebrüder Mönnig name, and the engraving says "Diamond", not "Diamant". That said, the engraving pattern does look familiar, but I can't remember where I've seen it before.
