Conn 30J DeLuxe Recording Model Bass
Date: 1934 (Drawing)
All I know is that these were produced around 1934. The 30J is the three valve version, the 34J is the four valve version.
What Conn said in 1934:
Wind bass players who double on string bass must have a bass easily accessible- mouthpiece and valves right where the player can swing over and touch them quickly, without hunting and fumbling. This bass was designed to meet this demand. The bell has been tilted slightly upward, the mouthpipe has been swung away from the bell stem, and a curved, swivel mouthpipe bit is supplied to meet the convenience of the individual player. The result is a comfortable playing position and unobstructed view ahead. This model has the same bore, the same big detachable, adjustable 26-inch bell, and other features that have made the 36J so popular. The intonation is even and accurate. Low notes that usually give trouble on other makes basses are particularly fine, coming out clear and full. Although the bore is large, the instrument responds freely and easily. BBb top action - 3 valves, 30J and 4 valves, 34J.