negative squared latin capital letter b symbol.
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symbol
B button (blood type) was assigned codepoint in Unicode 6.0 on 11 October 2010.
B button (blood type) was included in Emoji 1.0 on 04 August 2015.
🅱 U+1F171
:b:
🈲 Japanese “prohibited” button
🈷 Japanese “monthly amount” button
🈚 Japanese “free of charge” button