Date: 1928, Owner unknown
The 1926 catalog lists the 78H as a "Symphony Medium bore" with choice of an 8", 8½" or 8¾" bell. The bore was a #3½ which is 0.517"
What Conn said in 1926:
The Artists' model trombone presented here is probably the most popular solo instrument of the entire trombone family. It is generally preferred in the small bore, but it is also built in medium bore for those who desire a broader tone. The Symphony Models are exceedingly popular for orchestras, generally being used for first trombone. The medium bore Symphony is especially versatile, answering the purpose of orchestral work in any capacity outside the Bass Trombone requirements.