Postage Due Specimen stamps.
Specimen stamps came about as a result of the need for participating postal authorities to provide the Univeral Postal Union (UPU) with examples of the stamps they had on issue.
Until 1913, when Australia finally got around to issuing Australian stamps (the Kangaroo and Map Series of 1913), New South Wales, Tasmania, Western Australia and Queensland each distributed stamps to the UPU individually.
Western Australia cancelled their stamps with a four-ring concentric circle postmarker, thus:

You may note that these are a mix on the first series (space at foot of design) and the second series (design complete). Both these issues were distributed at the same time in July 1902.
Tasmania place a 'specimen' overprint vertically on the first series ½d, 1d, 2d, 3d, 4d, 6d and 8d along with the 5d, 1s and 2s of the second series.
I only have two, thus:

New South Wales placed horizontal overprints on all values of the second series only (the first series was not distributed to New South Wales), thus:

Queensland sent unused stamps to the UPU and these stamps are not easily distinguished.
Finally, there were stamps issued by the UPU to member countries. Portugal overprinted these stamps 'Ultramar' on receipt. I don't have any of these but have requested a scan from a collector I know who has then and will post when to hand.
I hope this of interest, I will post on another favourite stamp soon.