I have a badly mutilated first flight cover celebrating QANTAS's first Round-the-World flight, postmarked 14 January 1958 and backstopped on return to Sydney on 20 January 1926.
The cover includes a pair of SG 301 which was issued for the event. They are unremarkable other than a great deal of fly-specking on the plane's left-hand vertical tail surface. The accompanying 1/- Lyre Bird has similar markings in the lower left margin.

From the left stamp:

From the right stamp:
