Thanks to John (aussie240) for the following:

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The first pic is a collection of double adaptors which reverse the polarity of the L&N connections. In the second pic you can see why this happens.

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Thirdly, we have the inside of an old bakelite plug with L&N not marked; only the earth is:

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In the 4th photo is a later plug (1960's) which does have the polarity markings:

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