What are the words that John Boyne uses to signal this flashback in chapter 11 of the boy in the striped pajamas

While I have never read the book (because it is historically inaccurate - so don't get too sad, because it never could have happened), there are many words that would signal a flashback. Flashbacks are a time of reflection, so there are many past tense verbs used. You may also see things like, "Back in my day ... ", "As I think back ... ", "When I was a young boy ... ", "As I remember ... ".