Actually, if you look in to Autogyros, gyrocopters...whatever you want to call them & look at Juan De La Cierva as he had designed the first Autogyro. He had designed the flapping hinge and after it was first built it also suffered an engine failure and Auto'ed to the ground safely.
I would expect his designs were the first to allow a disengaging drive system from the pre rotative drive systems.
From there the helicopter the helicopter took off and most certainly had a clutch system incorporated along with the flapping hinge ..etc etc.
It was pre 1939 for sure though designs were always being copied and borrowed & improved.
I would look at the C30A (built in 1934 & designed earlier) closely for designs in the drive system as the rotor system was driven by aux drive from the engine.
From there you could look at Pitcairn (built under license from Cierva).
Prior to that (Cierva)...not sure, no one else was succesful in getting them to fly though maybe someone did think of the clutch drive .....but I would be surprised.
Now if you want to get anal you could think of Sikorsky as him having the first sucsessful helicopter design SV-300 in 1938-39.
Prior to him I would look at Frenchman Louis Bréguet and German Heinrich Focke as they had both heavilly contributed in the early 1930's to helicopter design.
You may get caught out when you read something that states somebody built something. Yes they may have built it but that doesnt mean they designed it.
HF