/usr/lib/x86_64-linux-gnu/libserial.so is in libserial-dev 0.6.0~rc2+svn122-4.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libserial.so.0.0.0
/usr/lib/x86_64-linux-gnu/libserial.so.0.0.0