This is a rear spring adjuster kit that is compatible with the height adjustable damper kit for the CIVIC FK8. The adjuster can accommodate changes in the free length of the spring and the spring rate, allowing you to adjust the vehicle height to the best possible level.
It is introduced on Ameblo's "M&M HONDA's Honda Honki."
https://ameblo.jp/mandm-honda/entry-12841865175.html?frm=theme