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Harry Campbell (a Cambrian, oddly, not a Caledonian) gave it this way in Whatever Happened to Tanganyika?:
'The terms "South-Brittains" and "North-Brittains" for the English and the Scots had already been coined at the time of the Union of the Crowns in 1603, when the kingdoms of England and Scotland were united under James "the Sixth of Scotland and the First of England" as "King of Great Britain". It seems fair to say that "South Briton" never exactly caught on.
. . . . . . . .
'[But] according to the Oxford English Dictionary, the term "North Britain" is "still in occasional (chiefly postal) use". Well, perhaps when those words were being drafted, nearly a century ago. The term seems to have been dimly remembered as late as the 1930s, to judge by an anecdote retailed by one H Teeling Smith, who sent a Christmas card from London to an address in Fife. Someone added the initials "N. B." to the envelope, and when the card eventually arrived—via Canada—it bore the words "Try Scotland". It had ended up in New Brunswick. In the time-honoured way, Mr Teeling Smith wrote a letter to The Times about it: "Let this be a reminder to us all," he thundered, "that the British postal authorities many years ago issued instructions that the letters N. B. must not be used as an abbreviation for North Britain!"'