Last time I played with installing an older version of calibre on Linux, the signatures for the older versions were in a subdirectory of the signatures called old.
Code:
https://code.calibre-ebook.com/signatures/old/calibre-4.23.0-x86_64.txz.sha512
I ended up downloading the txz file, checking the SHA manually and installing it using the command line from the calibre site (you'll need to modify the command line to the path/filename of the tarball.
sudo mkdir -p /opt/calibre && sudo rm -rf /opt/calibre/* && sudo tar xvf /path/to/downloaded/calibre-tarball.txz -C /opt/calibre && sudo /opt/calibre/calibre_postinstall