I recently purchased the cover in the picture and I am not sure I entirely understand the postage due charge in bottom left corner.
I think the letter had indeed not been franked enough, 2.5d was for destinations in Australia at the time.
How does it work from there? There is a stamp for 13 1/3 cent, this should be payed by the receiving party?
After that the barred stamp is put on the original T stamp?