Stamps with C. A. overprints are sometimes mistakenly referred to as being commercial overprints. In fact they are official government overprints and the initials stand for Crown Agents, who were basically the administrators of the British Empire from 1833 to 1974.
These overprints can be found on the Edward VII 1d scarlet and 6d purple and the George V 1d red 1911-1912. Overprinting was discontinued after this time.
The overprinted stamps were intended for fiscal use only, but are also known postally used. There is some variety in the positioning of the overprint on the stamp; typically styles h1a and h1b are noted.