We recently came across a C. G. Conn French Horn, in good condition. Few bumps and scratches, with a Conn Case. On the valves each has 3 2 1 respectively, then at the base of the center valve appears to be the serial number 129920, mouth piece reads Conn S. Bell reads "Made By C. G. Conn Elkhart IND." We know nothing about instruments. We have tried to search, only to be more confused. Could someone tell us more about this instrument and how much it could possibly be worth? We suspect it's a piston style, from early to mid 1900's at first guess, but it's just a guess. Thank you in advance.

Without a picture, couldn't tell you, but what it sounds like is a mellophone. If a horn mouthpiece does not fit, then it is indeed a mellophone. If it is right handed, it is 99.9% likely a mellophone. Not worth much, except for a very few models, which is mostly because of the curiosity factor. Look at horn-u-copia under conn and pull up the mellophone page. If it's one of those... There are some military piston valve conns from later on, and some really early piston horns that generally play pretty well in good shape, but they were all singles, and not nearly as popular or valuable as doubles.