Date: 1930. Owned by Paul Rawlins
A version of the 70H bass trombone was in production from 1920 to 1955. This particular version was produced between 1926 and 1937. It has a duo-bore: #4½ and #5, which is 0.547" and 0.562". The bell size is 9½". It also has an F attachment, and the F attachment has a rotary knob to turn the F attachment into an E attachment. The tuning slide isn't in the bell curve but in the hand slide, the same as the later version of the 70H. It can be adjusted by a screw in the hand slide. The advantage of this is that the bell can be completely conical while leaving the cylindrical tuning slide in a spot that is cylindrical anyway. The 70H was the large duo-bore bass, the medium duo-bore bass was the 14H.
What Conn said in 1930:
Most bands today use bass trombones, especially the larger symphonic bands. They serve to fill out the trumpet and tenor trombone ensemble by furnishing a rich bass of similar timbre and quality which blends well.
The new duo-bore feature in the bass trombone results in a new quickness of response and smoothness of scale that is a revelation to bass players who have never tried this model trombone. This effect comes from the greater degree of taper resulting when the slide nearer the mouthpiece is smaller than the slide nearer the bell. The 70H model is Conn's most popular bass trombone model, the bores being .547 and .562. Built in Bb with rotary valves to F and E. Tuning in slides; 9½" bell.