Postal rates decreased as of 1st October 1923. Instead of the more usual piecemeal revision of various colours and denominations, the magic date 1st May, 1924 saw a revamp of all values of the Kgv and roo series. At this time, the 4½d electro replaced the 5d.
4½d & 5d values are mutually exclusive. In their respective time frames, they were used to prepay the combined postage and registration fee (3d + postage). The 5d brown was dropped during the 4½ds reign, and, in turn, the 4½d was dropped when the 5d was re-introduced in 1930. The FIVE PENCE on 4½d overprint was a direct result of this.
Similar to the 1/4d introduced in 1920, this value is comparatively simple to follow having a single plate (not including the aborted ash overprinted 5d plates)