Conn 10C New Wonder Eb Soloist Model Alto
Date: 1918. Owned by Rich Wakefield
I suspect that this instrument, for all intents and purposes, is a Eb "Alto" cornet. In other words, I suspect it is a cornet in Eb, tuned lower than a regular Bb cornet. This would make it somewhat similar to the Eb Alto trumpet, the 34B. The 10C had a #2½ (0.484") bore, and was produced between at least 1919 and 1930.
What Conn said in 1924:
The Soloist Model was originally created in the Conn factories. It is similar to the cornet and only slightly larger. Its tone is unusually rich, and its peculiar color adds value to any combination of instruments of which it may be a part. The Soloist Model has also met with remarkable success as a solo instrument.