To continue, the brown, 20-Sen, Japanese stamp is in superb condition! It was cancelled with a light(possibly faded with time) CDS cancel from the Japanese Sub Post Office in Nishi Dai Mon, Seoul.
Lastly, as for the envelope, itself, it's in great condition! It's very clean! The back flap is fully intact, though there are a few creases in it. Very minor imperfections aside, this is a really nifty piece of Korean postal history!