What's the meaning of 'ER' and what will be the symbol of King

The cypher is the personal seal of the British monarch and like a host of other things will need to be updated for the new king.

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Remembrance Sigils (ER), Honkai Impact 3rd Archives Wiki

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The ER emblem, what it means and will it now change under King Charles - YorkshireLive

People are only just realising what the different postbox symbols mean and they're rushing to check their nearest one