Re: Mystery Alto
I would venture to guess it is an older East German replica of a Conn.
It has the Benz type cage on low C and the octave key mechanism of a New Wonder conn model.
Conn was rolling the tone holes at that time, so the soldered tone holes tell me it is a more primitive maker. Conn also was using mother of pearl rollers. Conn was the company to watch prior to 1939. After that, Coleman Hawkins and the Selmer promotions started to changed everything.
So, I would guess it is from the 1930s, pre WWII replica. Possibly and older Kohlert or Keilwerth early production horn. The round G# Pearl key is a pretty primitive design. It probably will not be the best sound out there, but it looks stable.
Maybe it is a demo of things to come from a maker, a prototype, as they learned from Conn.