Date: 1921. Owner unknown
The "Duo Bell Model" was produced from 1920 to 1925 and came with both a 6½" and an 8" bell as can be seen in the picture. It was a #2½ (0.485") bore.
What Conn said in 1924:
The Duo-Bell is designed to meet the needs of those who use the trombone for both orchestra and band work, the interchangeable bells making it possible to have the proper instrument for each kind of engagement without the expense and trouble of two complete trombones.