This era is listed by Conn as lasting from 1929-1931. It seems to me however, that these were probably made for Pan American instruments until the E-Z Tone mouthpieces came out. According to Conn's article in the January 1969 issue of Conn Chord magazine, this was in 1950. However, the 1956 Conn accessories catalog lists Pan American All Star mouthpieces, but doesn't mention E-Z Tone mouthpieces at all. So I suspect the change to the E-Z Tone mouthpieces actually happened around 1958, which was another year of significant change.
Quoting the January 1969 issue of Conn Chord magazine: "The All-Star mouthpieces were made by the Pan American Company, a division of Conn, set up to manufacture and sell modestly priced instruments, and accessories, particularly to schools. The "All-Stars" were easy to blow, being designed to aid the player in achieving a full, easy tone range. The cup had a broad flat rim designed for comfort and help in intonation." (Copyrighted, 1968, By Conn Corp.). I have never seen one of these.
The 1956 Conn accessories catalog has a table of Pan American All Star mouthpieces.
|Pan American All Star mouthpieces|
|156||Baritone (or Euphonium)|
|160||Bb Soprano Clarinet (Lucite)|