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postcard
memo; note; card
「端書」 has another reading with different meanings.
旅先から端書を出した。
I sent a postcard from my trip.
端書に絵を描いて送りました。
I drew a picture on a postcard and sent it.
端書の切手は六十三円なので忘れずに貼ってね。
Postcard stamps are 63 yen, so don't forget to put one on.