Find the exact Build Number of a specific Mac OS X update from the command line
The relevant file for that specific dmg is SecUpd2020-006Mojave.pkg/PackageInfo.
After mounting the dmg, do this:
% cd /tmp
% xar -x -f /Volumes/Security\ Update\ 2020-006/SecUpd2020-006Mojave.pkg SecUpd2020-006Mojave.pkg/PackageInfo
% grep -o 'com.apple.pkg.update.*&quot; ' SecUpd2020-006Mojave.pkg/PackageInfo | sed 's/.*\.//'

For updates that have been automatically downloaded to /Library/Updates/, the relevant file is named *.dist. For instance, in 001-48382/001-48382.English.dist there is
&lt;key&gt;macOSProductBuildVersion&lt;/key&gt;
&lt;string&gt;17G14033&lt;/string&gt;

and also
com.apple.pkg.SecUpd2020-005HighSierra.RecoveryHDUpdate.17G14033