A while back I noticed that the initials IRB appeared below the portrait of the Queen on recently-minted coins. It turns out that those are the initials of of Ian Rank-Broadley, the man who drew the portrait. I found that courtesy of the Wikipedia article on the British one penny coin.
So now you know the reason for those letters. I thought it might have had something to do with the series of the issue or something. But it was much simpler than that. It seems a bit odd that whoever is in charge of choosing the image allowed the initials on there. It seems weird for the artist to sign coins. But then, our notes used to have © Thomas De La Rue and Company on them (the paper notes).
IRB also did the Australian coins Queen portrait. All of the old NZ coins used to have JB stamped on the front somewhere, oo, just checked the new NZ coins and they do as well. The 5c echidna in Australia was drawn by JD, the 20c platypus is a natural phenomenon.
True, I forgot to look at the front of the coins. The new 20c doesn’t have anything as far as I can tell. Can’t find anything on the 2 dollar coin. Perhaps the ones designed for 1990′s issues weren’t credited like that. I wonder if any non-Commonwealth countries have credits like that, particularly euro countries.