This chapter's historical lore (especially Taisho-era Mary-san) builds on what has previously been shown way back in Ch. 19 where if phones, being a new-fangled invention of the time, weren't widespread she'd attempt to hand-deliver a letter instead. Also the sweet and creamy pudding we know today dates back to at least the mid-18th century around 1755 meaning according to the last panel Mary-san herself may predate her most treasured treat!