I believe what follows is an accurate description of this mouthpiece. It was often difficult to discern the difference between many similar looking models. I found the Theo Wanne website the most helpful. This is a classic Selmer mouthpiece of (I think) the 1950s. Some of the information indicates that the mouthpiece design actually dates back to the 1920's.
They had a short, almost stubby, shank and the name Soloist engraved on the table. The first of these had an almost oval chamber, much like the Brilhart Ebolin and Tonalin Great Neck NY mouthpieces.
There was a small flat at the back of the floor (front of the chamber) in their molds. This flat was enlarged by hand to make it more oval.
Later, the hand work to oval this section was not done. Finally the chamber became truly horse-shoe (or dome) shaped. The Selmer Soloist short shank hard rubber models came in sizes from B to H.
For reference an E is equivalent to an Otto Link 5, and an H is equivalent to an Otto Link 7. The Selmer mouthpieces from B to E all came with a medium length lay (the distance from the tip of the mouthpiece to the point where the reed meets the facing). This mouthpiece has been in my possession, I believe, since the early 1970's. I also believe that it came with my Selmer Mark VI alto sax that was provided to me by the U. Air Force during my time in the service (band).
It has had very little playing time as I used another C as my primary mouthpiece. My principal instrument is clarinet. The item "Selmer Alto Saxophone C Soloist Short Shank Mouthpiece Vintage 1956" is in sale since Tuesday, July 3, 2018. This item is in the category "Musical Instruments & Gear\Wind & Woodwind\Parts & Accessories\Mouthpieces".The seller is "jijiml7dnd" and is located in Fort Mill, South Carolina. This item can be shipped worldwide.