According to yugoslavian sources in 1934 japan consul sent 372 trees of unshiu, honikan and natsu mikan as gift to Yugoslavian kingdom.
This might be off-topic, but I have tasted fruit from a tree that was labled "Natsu daidai". From research I did, I believe this might be the same thing as Natsumikan.
The fruits are big in size. The fruits are like a mix between orange, grapefruit, and sour orange, and have just a little bit more bitterness than a grapefruit. I was able to enjoy eating it, but I'm not sure if everyone would. Certainly something different, a delicacy. The flavor and aroma were reminiscent of Jamaican Ugli fruit (another type of grapefruit). It would probably make great marmalade too.
It's also worth saying that, unlike with a normal grapefruit, the white pith is sweet and not bitter. They make candied rind out of this fruit in Japan.
The fruit contains a fairly high number of seeds.
The original story in Japan is that this Natsumikan variety originated when a woman on the East side of Japan found a fruit washed up on the sea shore, and grew the seeds.