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).