Date: 1930 (Drawing)
Not the best quality picture. The 14L was produced from about 1926 to at least 1930.
What Conn said in 1930:
The One-Valve Tenor Trumpet [is a] new instrument added to the Conn line. [It] plays tenor and bass parts and the use of [it] greatly increases the range of the bugle corps. [It is] built in regular three-valve trumpet style, in Bb with valve to F.