Kubo playing with the words again.
Kubo wrote the chapter number of 665 out as 六六五, which is unusual; it should read either 665 in arabic numerals, or 六百六十五 in kanji.
There's no furigana to denote it being read any differently than it appears, so it actually reads as "Roku Roku Go"(Six Six Five) and not "Roppyaku Roku-juu Go"(six Hundred Sixty Five).
This is weird because Kubo typically writes the numbers out in arabic numerals, like every other manaka, and the last time he played with using kanji numbers it was in "Baby, Hold Your Hand 七" but that was a play on 七 being read Nana, and the reveal that Nemu's full name/label was Nemuri Nanagou(眠七號): "Sleeping No.7"
In ch 608 title too it was in kanji, referencing Ichibei's calligraphy.