Could it be a maundy die that was later used for currency? The annual reports show that there were 43 obverse dies used for 1,469,592 currency 3d pieces and 3 obverse dies for 7,704 maundy 3d pieces. If just one of those dies contained the error, it's unlikely that there were originally more than 10 or 20 thousand RRITANNIAR minted.
It does seem unusual that the error was not corrected, given the number of B over R in BRITTANIAR that exist in other denominations.