Do you know what Version was used to write the data to tape?
You could install the latest BE 20.3 trial and install it and inventory/catalog.
If there is some data\backup sets agent options that is EOL on the tape then they will not show in the restore selections.
What type of tape cartridge is it? 15 years ago there was a wider variety of cartridge types and styles.
What type of tape drive was used? brand? Model?
Do you believe that you still have a working tape drive that can read that cartridge?