You've got a double strand of vintage Japanese akoya pearls. The graduated style was very popular after the war, as GIs brought a lot of them home. Most were single, 3.5 momme strands. This strand might have been two singles at one time as it's been individually knotted throughout. Most of the older 3.5 momme strands were only knotted near the clasp. The clasp and the end caps are vintage though, so it may be original.
They could be Mikimotos. At the time, Mikimoto was the largest producer in Japan. I don't have a lot of experience with vintage Mikimoto boxes, but the ones I've seen are stamped on the inside of the cover. There should be a hallmark on the clasp if it is a Mikimoto and the original clasp with the pearls.