The purchase of post office packs of cancelled to order stamps of the current series was quite popular at the time. In the case of the higher values, these were overprinted 'specimen'.
Of interest, the 10/- and 1 pound values appear on both white and cream papers for reasons outlined in the beginning page. This would confirm that subsequent print-runs were required for these in-demand parcel values, as well as a requirement to re-stock the popular specimen post office packs at the same time. The post office was not concerned, nor indeed aware, that the papers had changed to a marked extent, the new white paper varieties were simply a result of fresh orders required, not, a deliberate policy of providing additional (different) specimen sets.
The overprint was performed on the single-pane post office sheet of 80 stamps, not the printsheet. The overprints themselves originated from quite ordinary individual electrotype lettering. As such each overprint on the sheet is marginally different. The forme used was made up as required and not kept between print runs.
The height of specimen overprints was originally 15.5mm high, and occurred only on the initial cream paper print run of the 10/ Flinders. It was subsequently altered to 13.5mm for all other values and papers, including fresh printings of the 10/- on white paper.
Easily the smallest PO issue of anything Australian since WW2. First printed in September 1964, new ACSC research shows only 3,480 sets of 4 were ever supplied by Note Printing Branch to the Post Office, and many believe not all those were sold in the 17 months before the Decimal issue of February 1966 and were later destroyed.
10/- and £1 scarce WHITE papers, with the 13.5 mm overprint – appeared in sets only in November 1965 – 3 months before being made obsolete