You need to find where your custom tab is stored.

Least likely: normal.dotm

It could be in the Word.OfficeUI file or in a separate template. The Word.OfficeUI file is not a file that transfers to other systems and includes default-location QAT modifications.

You can export ribbon modifications within the Word version that created the Word.OfficeUI file. That export file can be imported into another system. I have never tried doing it between versions.