Vocal horns are essentially tenor horns pitched in F that use a "French" horn mouthpiece. Note the Art Deco styling for the period (1930s): rimless bell, stylized spit valves, geometric valve casings, elaborate engraving, heavy gold-tone lacquer finish.
It measures 24.5" across from the end of the mouthpiece to the farthest edge of the bell. full-sized (4/4 size) instrument is nickel-plated, and features three piston valves.
That way you can get any backbore combination you want.
Bach should wake up and revamp the entire line of trumpet mouthpieces.
The bell of the instrument measures 11" in diameter.