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.*" ' 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

<key>macOSProductBuildVersion</key>
<string>17G14033</string>

and also

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