Action Group.

Before you take the effort to find the correct folder (here is a thread that deals with backup and installation: /t/964/1
you might open the mta-file with the Windows Editor and see if it really contains the actions of your action group.

Places that might also contain mta-files:

  • the installation folder for MP3tag
  • the program folder of a portable MP3tag installation.

As you found the standard.mta files in the %appdata%\mp3tg\data\actions folder it is fairly likely, that you have a portable or migrated installation that does not use that folder.