I saw this today on eBay. The Conn Constellation is a fine instrument, probably the best of the older curved-bell American horns. It has a medium-size shank, 4 front valves, and a tuning trigger on the main tuning slide. Tone was very nice as I recall. Henry Charles Smith played one of these and sounded great on it. I have tried them before and liked them.
The horn won't sound like a modern compensating horn and has a smaller sound than most of today's top instruments. But it's a nice instrument with a pretty sound.
I don't know anything about the seller or this particular serial number instrument, so be careful (as always on eBay).
Conn Constellation Euphonium