Here's a neat one that scans well to see the features. This is Scott 103, ACSC 138, block of 4. I'll show a "normal" stamp, followed by the variety ACSC 1383, Re-entry to swan's neck and "T" of Australia.
The most obvious feature is the re-entry in the "T" and how it clearly stands out. However, please note the marks in the "R" and the additional marks in the "W" of Western just below.
And, as described earlier, please note the extra curve in the swan's neck. There are also some extra marks in the upper back, just below the curved neck.
Note that there is also a slightly different Re-Entry, ACSC 138f that has lesser markings in the same places. If anyone has one of those, please post it. Thanks.