Inspired by a letter box

Ever since, when I was a child, my father drew attention to letter boxes emblazoned with ‘VR’, I have been on the lookout for more examples. Queen Victoria’s 63 year reign ended in 1901, but it is not uncommon to find a ‘VR’ letter box still in use. George VI letter boxes, on the other hand, are less common, as he was only king for just over 15 years. He was followed by the late Queen Elizabeth II who reigned for 70 years.
I wonder how long it will be before I spot a box emblazoned with Charles III?