Date: 1924. (Drawing)
The Wonderphone altos were the 6C/7C (pictured here) which were top action instruments, and the 8C/9C which were front action instruments. The 6C/7C had a #3 (0.500") bore and was produced from at least 1919 to 1925.
What Conn said in 1924:
The type of Alto illustrated on this page [is] most popular for marching and other outdoor engagements. The Wonderphone model has been designed to serve as well in concert work, for the curved and adjustable bell makes it possible to throw the tone in any desired direction. Conn Altos are unequalled for brilliance of tone and ease of blowing because the taper branches are shaped to exact dimensions by the patented expansion process, and the valves are quick acting and smooth in operation because they have been designed and built after fifty years of experience in this work.