Date: 1967, Owner unknown
Models after 1948 don't have (generally thought to be superior) rolled tone holes but (inferior) drawn tone holes. Apparently rolled tone holes are more difficult to manufacture. The 12M was probably never available with sheet metal keyguard. (source: Saxpics).
What Conn said in 1959:
World's greatest players choose this baritone saxophone. Easy to play, easy to handle, the "total tone" built into this saxophone is unequaled for smoothness and response. Engineered hard nickel key design with longer hinges gives speed and comfortable "feel" that will stay in fine adjustment for a long time. Features that only Conn can offer: hydraulic expanded tapers, Durocast keys finished in tough long-wearing nickel, drawn tone holes, Res-O-Pads, adjustable pivots, LUSTRE-CONN finish.