And a little bit of an update to this - I haven't done a in-depth study of the G with rock/G with no rock from 1861 to 1873, but I have had a brief look.
From 1863 to 1873, the pictures I have at hand all seem to show a rock (though a bit faint at times).
More interestingly, the 1862 with A to left of lighthouse has no rocks while the 1862 with A to right of lighthouse has a rock, at least from the handful of images I've looked at (https://www.baldwin.co.uk/product/victoria-1837-1901-halfpenny-1862-die-letter-a-rrr/, https://www.londoncoins.co.uk/?page=Pastresults&auc=124&searchlot=555&searchtype=2, https://www.londoncoins.co.uk/?page=Pastresults&auc=162&searchlot=2355&searchtype=2).