Australian Antarctic Territory, Air Mail December 1959

I purchased this in an estate sale about 4 years ago and do not know much about it. There is a stamp on cover No 248. What is that stamp? There are 5 cancellations. On the stamps is the 8D and 5D Australian Dollars? Anyone know what the exchange rate in 1959 USD to ASD? I find this very interesting. Any info would be great.

Does this cover have any monetary value? I value it as a piece of history.

This cover was mailed before the Australian dollar was established and that makes a conversion into American dollars a little difficult. If I understand things, conversion of this amount into American dollars would have given approximately 56 cents. (I hope someone will correct me if I'm wrong about that.)

Incidentally, the 'd' you see on two of the stamps stands for 'penny.' (Why is a long story.)

The "d" stands for dinarius which was an old Roman coin. In England "d" was used to stand for "penny" or its plural "pence". The "/" in 1/- & 2/3 stands for shilling. 1/- = 1 shilling (or 12 pence). 2/3 = 2 shillings 3 pence.

12d = 1/ (12pence = 1 shilling). Continuing the lesson: 20/ = £1 (20 shillings = 1 pound). This was all pre-decimal era which ended in 1971.

The "248" is the number of the registration label and is not a stamp

The four stamps add up to a total of 4 shillings and 4 pence. I shilling equals 12 pence (d). (The 2/3 stamp is two shillings and 3 pence).
When decimal currency was introduced to Australia in 1966, one shilling (12d) equalled 10 cents. So in Australian currency, the total value of the stamps was about 43 cents Australian. In 1959, the government regulated the value of the dollar and I believe at that time it was worth more than the American dollar. When the Australian dollar was floated in the 1980's, its value has fluctuated a lot against the American dollar. At present, it is equal to about 73 US cents. About 2 years ago, it was higher than the American dollar.
The R stamp you mention is a registration label.

I found a pre-decimal inflation calculator for Australia and, all things being equal, the four shillings and four pence registered letter sent in 1959 would cost AUS$6.23 in 2014.

Registered mail was started in the UK in 1841. Markings on registered letters and parcels used to be green ribbon tied around the letter or parcel, then it became green string, then it became pre-printed blue lines to imitate the string or blue pencil lines drawn to look like the string, then it became small labels such as the one on your letter. Registration is an extra cost above and beyond the regular postage rate and it provides extra services comprised of proof that you mailed the letter and when it is delivered proof that it was delivered, and in between it is kept under greater security than the regular mail. It is mostly used for sending important papers or sometimes cash or such where you really want to be sure your letter or parcel arrives without problems. Each post office kept a ledger book in which they would note the sequential registration number and details about the sending of the letter or parcel. The number served as a kind of internal control number within the post office. As the others have said, these are not stamps, they are simply labels supplied by the post office when a sender paid the extra fee.

Your cover was mailed at Doutta Galla which is just outside of Melbourne, Australia. This one, and ones that were mailed from the post offices in the Australian Antarctic Territories are relatively common and sell on ebay for $1 to $2 but I think they are really nicely made and fun to collect.