To even the most casual observer, there are two classes of stamps that are startlingly different in appearance.
The photogravures 1st appeared for the 1956 Melbourne Olympics, and were introduced very, very late in the pre-decimal era. Within a few short years after decimalisation they took over completely.
Of the three major printing techniques useable at that time, typography, recess, and lithography, only the first two were employed on Australian pre-decimals (see note). The major visual difference between the two being that in recess, the image is impressed into the paper, giving much sharper detail. In typography, alias letterpress, the image is pressed onto the sheet, relying on the ink to take up onto the paper.
From the engravers perspective, the lines he cut in the die for recess issues, were the lines of colour on the stamp. For Typography it was completely opposite, he was creating a true negative.
Two quite different methods were used for monocoloured printing in Australia.
Note: Lithography, literally stone printing, was used on colonial issues (WA 6d lithographed swan e.g.) but not in the Commonwealth era.
This is a dual plate printing using Recess (green) and TYPOGRAPHY for the lilac.
Prior to the Chambon multicolour photogravures, dual color printing was used sparingly for only a few commemoratives in the 1950's. Clearly however, a fore-runner of things to come.