Thanks for that Blair,
I still have to do homework.
According to a West Aussie Airways specialist (not routes)
http://cjoint.com/data/guiubFUxmf.htm

Which explains the trip of mine 3d each way Carnarvon to Perth
then Perth to Nullagine.
However plain mail back to the addressor was free?

I feel "Transwest" aust probably took it from Carnarvon to Nullagine
then ordinary mail back to Carnarvon
which has the AR card over franked.

A nice little mystery to solve.



"Blair"
The pink cards were stapled or taped to the back
of the registered letter. They were removed upon
delivery, stamped , and mailed back to sender.

Later the pink cards had a detachable gummed tab
along one end of the card. The gummed tab was used
to stick the card to the registered mail.

Upon delivery, the card was removed (there were perfs
between the card and the tab to do this) while the tab
was left on the registered cover.