I found an older thread with some good information from 22crows. There is a good website with additional information on this topic, if anyone is interested:
http://www.ozrevenues.com/Revenue-R...atalogue.htmI just purchased (2) of the items which are identified as 1918, 4th Series, Plate II, Perf 12 1/2. What I find interesting is the upward shift in a bolder "E" in overprint Cent, with the addition of a break in the "O" of One.
A "regular" stamp:

And the unexpected version:

The website doesn't go into any details on variations in the overprints, but if anyone out there has something similar, please post a copy. Thanks.