A reasonable “fallback” mechanism to find the prior version, if done via “Upgrade Automatically” ( the classic “oops”), is to examine the war file location. The prior jenkins.war file should have been renamed to jenkins.war.bak.
You can either find the SHA:
sha256sum jenkins.war.bak
and look that up in the Past Releases, or you can run this:
jar -tf jenkins.war.bak | grep jenkins-core
and the resulting number will be the release.