I have seen a stamp catalogue indicate this stamp
is described in error, the "throne" being the "Coronation chair".
It appears <this> is in error also, as the chair is indeed used as a throne.
Your stamp (letters=BG) is Stamp Number 27 on the 50 stamp plate.
It is suggested Thomas Ham the engraver, organised the lettering
so that Queen Victoria ("QV") was on the bottom left corner stamp,
the letters going from and back from that stamp
Your stamp :

The plate with "QV" in bottom LHS position :

Acknowledgement" Mr. Geoff Kellow
The Australian Philatelist March 1988