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It's a mechanical watch (autowind) with rotating gears. The left hand thread may keep the screws from backing out over the years from directional micro torque or vibration. They may even make all of them left handed; so, same size screws used for different purposes in the watch will be interchangeable and can be modified to different lengths.
It might be somewhat like one bicycle pedal being reverse threaded, even though the pedal rotates on its shaft, so it won't eventually back out. That's my guess.
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