Is someone able to explain the subtleties of difference
between a common "Registered Letter" and this "AR" (Avis de Reception) card?

Carnarvon Western Australia to Nullagine Western Australia 6d via air
(assuming Trans West Airlines)

Nullagine's population is assumed low (100-200?), due to the single name
adressee.

http://cjoint.com/data/gtgsZaLU0Z.htm
http://cjoint.com/data/gtgxSssAfS.htm