a character for an unknown character

[David Corbett]

One Unicode character specifically for this purpose is U+3013 GETA MARK. It
is a Japanese symbol used to replace characters that cannot be read during
transcription of manuscripts (source: Japanese Wikipedia). It looks like a
bold equals sign: 〓.

Other people have suggested U+FFFD REPLACEMENT CHARACTER, but that is the
diamond with a question mark inside it that you already mentioned.
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