By the middle of the 20th century it was realised that the letter I was easily confused with the figure 1 and Q with O and 0, while odd codes had been introduced with the small letter o tagged on. So the system was revised, eliminating the o codes, and extra two-letter codes were allocated: CM De Aar, CN Wellington, CO Calitzdorp, CR Hopefield, CS Bredasdorp, CT Ceres, CU Port Nolloth, CV Vredendal, CW Worcester, CX Knysna, CY Bellville and CZ Beaufort West.