This is record TInt64Rec.

public TInt64Rec = packed record
  case Integer of
    0: (Int64Value: Int64;);
    1: (High: LongWord;
        Low: LongWord;);
    2: (Words: array[0..3] of Word);
        Bytes: array[0..7] of Byte););
  end;

ImagingUtility (at line 84)