pete said:The F Mezzo-Soprano and Alto
The Conn-O-Sax, which has a bigger bore and longer keyed range, is a "true" alto and has a darker tone than the Conn F Mezzo. It's also considerably more rare.
Actually, The Conn F mezzo-soprano and the Conn-O-Sax are exactly the same instrument from the low D key on up. Same bore, same mouthpiece, same neck, same keywork. The Conn-O-Sax has high F# and high G keys, which the F mezzo usually doen't have, but a few F Mezzos had the high F#.