This is the replacement spool holder on Daiwa SV reels and SV spools sold from 2013 to 2019. It is a cam at the brake rotor base, to push out the brake rotor when it is twisted by magnetic force. Its older version in black tend to get stuck with rotor cam, or to break in time at its very thin posts.
This white one is reinforced parts to replace the black spool holder. Later than 2020 year model SV reels seem to have new black cam in better design.

The C clip holding rotor may be removed by 2 flat head scerw drivers. You'll surely need a tweezers to set it back, to hold a washer in position and to set the C clip in the groove. C clip and a washer are pushed by a spring, and they jump out. Please take care.