/usr/lib/x86_64-linux-gnu/libnuclient.so is in libnuclient-dev 2.4.3-3.3build2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libnuclient.so.4.0.0
/usr/lib/x86_64-linux-gnu/libnuclient.so.4.0.0