I’ve just finished Day 1 of the book and am starting Day 2. The twist wasn’t quite what I was expecting, but was actually even more interesting than what I was anticipating. The specific question, which I don’t think deals with any major plot points, is this. What time period is the story set? From what I can gather from the clues from Day 1, it seems like it’s set some time in the period between WWI and WWII. Maybe a little before WWI if I’m off about some of the references. Please spoil this one aspect of the book, but not others. Thank you 
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IMO, it’s pretty indeterminate, but about late 1920s seems right to me.
I’m not sure about the late part, but definitely 1920s.