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I don't know Why but I think master Sirius is Vincent Phantomhive. Because that hand was burnt... (⊙…⊙ )
Excuse me for my bad English.
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I mentioned the top of the arm. it's black like burnt. When Sirius called Violet "Come here child. See
http://www.mangago.zone/read-manga/kuroshitsuji/an/v23/c115/pg-23/
Wasted my precious time on this ( ̄へ ̄)