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It might be something called a "pendulum detuner".  The are designed to partially damp out the worst natural mechanical resonance frequency of the cable.  Wind can excite resonance in a cable, which causes much higher stresses than just the normal hanging due to gravity.

I found <a href="https://plp.com/energy/transmission/motion-control/detuning-pendulum">an informative web page</a> from the PLP Company about these products.  That page also has a video explaining the problem and how pendulum detuners address it.  Here is an excerpt from that page with the basic info:

<img src="https://electrical.codidact.com/uploads/qswoaxsacb0256pjyavk11jcow25">