"The 6-D is a double horn with rotary valves, with an extra piston valve to enable the performer to select either side instantly, thus having an instrument in either Bb or F." (C.G. Conn brochure CD 201 - 15M-2-26 presumed to be ca. 1926)

The above illustration is from the cover of the 1926 C.G. Conn New Wonder French Horns brochure. The model 6-D (later used to designate the company's all rotary-valved model) was a direct copy of the C.F. Schmidt double horn with a few modifications: third B-flat valve slide is the more common tear-drop shape and the legs on the main F tuning slide (to the bottom of the picture) are reversed. This is the only double horn in this brochure and presumably the top of their line intended to compete with the then popular C.F. Schmidt imported by Carl Fischer, Inc. and the Kruspe imported by Anton Horner in Philadelphia (later copied as the Conn 8D). The brochure includes endorsements by Louis Dufrasne, principal horn of the Cleveland Symphony, and William Frank and Max Pottag, principal and second horns respectively of the Chicago Symphony with the inplication that they used these horns in their profession. Howard Sanner provides an excellent and very detailed description of the Conn Schmidt Copy. Another example owned by Leonard Brown can be seen at the The Conn Loyalist webstite.