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There are simply thousands of Pillar Boxes across the United Kingdom and the best way to date them is to look at their cypher. This will identify the Monarch that was on the throne during their creation. When the monarch changes, new pillar boxes do not replace old but are added to those in use, and this is why Britain has such an array of boxes.
You will find, however, that in Scotland pillar boxes do not have ‘EIIR’ on them. This is because some Scottish people did not accept Queen Elizabeth as the second monarch of her name, since Queen Elizabeth I was never ruler of Scotland. Earlier pillar boxes with the cypher were vandalised and even blown up, and because of this they instead depict the Scottish Crown.
They're increasingly rare - which makes sense, given they have to be 100+ years old by definition. But they're out there.