>>18896848 lb/pb Durham [B] killbox
Probably not a killbox, Anon. It's used at the start of a quote. It's a simple letter case change in a direct quote. '[B]y the date…' The brackets indicate that the author has changed a lower-case 'b' to an upper-case 'B' in a direct quote.
"When writers insert or alter words in a direct quotation, square brackets - [ ] -are placed around the change. The brackets, always used in pairs, enclose words intended to clarify meaning, provide a brief explanation, or to help integrate the quote into the writer’s sentence."
"Integrated quotation with brackets used correctly to indicate a change in letter case:
(Example sentence):
Salvucci and Taatgen propose that “[t]he heavy cognitive workload of driving suggests that any secondary task has the potential to affect driver behavior”
"Note: Brackets are placed around the lower-case letter ‘t’ to indicate that the letter case has been changed. The quotation is introduced by a signal phrase, which makes the quote an integral part of the writer’s sentence; as a result of this syntactical change, the upper case ‘T’ in the original is changed to a lower case letter."
https://writingcommons.org/article/inserting-or-altering-words-in-a-direct-quotation/