Nick Rowland from Ohio USA directed me to information on his 1904 single cylinder tractor with a group 7B desmodromic valve.
How the desmo command works:
One cam works a roller on the top of the exhaust rocker arm to open the valve and another cam & roller on the bottom of the exhaust rocker arm pull the exhaust valve closed.
The rocker arm is balanced on the valve by a stiff spring, so the only thing holding the exhaust valve closed is the rocker arm & cam.
How is it classified:
There is no name on the engine, but it is stamped with a serial #24.[Unknown / uncertain manufacturer (possibly a Frank Van Duzen)]. It was built locally and Mr. Rowland was able to accurately date the engine by the serial # of its ignition magneto which dates to 1904. Mr. Rowland may be regarded an automotive historian and he possesses a vast collection of tractors and stationnary engines.
[Classification = Group7B]