I just happened to come across your post, better late than never!
I think you were involved in her towage from Tasmania to Singapore in 1978, following her three months long make-over at Bell Bay.
I understand she underwent below the water line hull repairs (used a cofferdam) as well as pipeline firing line equipment upgrades and other topside additions (e.g. office and accommodations added under helideck), and rearrangements below deck in her public spaces (e.g. cinema added), among other work items.
I attach the two photographs I have of DB21 in the Tamar River. These pictures were published some years ago by Glasgow Engineering Co. Pty. Ltd.