Update: Livery button by Stokes & Martin
The style of “crown” is a Baron’s coronet. A baron’s coronet has six ‘pearls’ of which 4 are visible when drawn.
As the button was made by Stokes and Martin, I presumed a Baron living or visiting Melbourne ordered this from 1862-1893. Scanning through newspapers of the period, I came across 3 Barons although there may have been more: Frederick Von Mueller,Charles Robert Carrington (Governor of Sydney from 1885-1890, and John Adrian Louis Hope (Victorian Governor from 1889-1895).
This turned out to be a misdirection, as the button is the livery for Lord Brassey who followed Hope as the Governor of Victoria from 1895-1900. He must have ordered the buttons before 1893, or more likely, the buttons were produced with old backs.
The button is in error. It should show an Earl’s coronet, as this livery button shared on Twitter by Anne Brassey shows:
Even this button is not correct; the Earl’s coronet has eight ‘pearls’ of which five are meant to be visible. This button may have been trying to show all eight. The bird is a mallard (i.e. a duck).