You may wish to have your 3rd stamp down, checked.
It may be a forgery.
(I am no authority on early Australia)
"Queen on Throne" (That was in error, it actually is the Queen on the Coronation chair)
Plate letters YC appear on the plate 3rd row down, 4th from left.
The floral emblems top left and right look dodgy.
Here is a genuine one (I think) with "Piecrust" Postmark
(Image may belong to Brian BC ?)