Keith,
Desmodromic is slightly different and doesn't rely on Pneumatics. The cams both open and close the valves.
Desmodromic principle was originally designed in 1910 by an English Engineer called Arnott.
I'll send you a picture on email
1956
Assistente tecnico (Engineer) Taglioni leads the way to the Ducati desmodromic system: for the first time, a desmodromic timing system with three overhead camshafts, driven by a vertical shaft and a bevel gear, is mounted on a 125 Grand Prix bike: this bike and its rider Degli Antoni win the first race, the Swedish GP.
From now on, the desmodromic system becomes a Ducati exclusive.
1968
To see the first mass-produced desmo, we will have to wait until the Mark 3 Desmo 350 in 1968. This bike has practically the same timing system as today's twin-cylinder, two-valve bikes, which is evidence of its exceptional efficiency.