= Solution
The orientation-dependent results in the official notes show that the measured <spring constant> is \b[not a Lorentz scalar]. A <Lorentz transformation> changes both the simultaneous extension and the measured three-force; their ratio depends on the orientation. This does not make the spring intrinsically anisotropic in its own <rest frame>, or require a preferred inertial frame. It is a quasistatic effective stiffness comparison between frames; an extended real spring also has finite internal signal speeds.
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