Prunus #29 what kind?

This is a very old collected tree that I have acquired in Japan. It may also come from Korea. They sold it to me as Prunus ume. But looking at the tree now I don't think it is P. ume. The blossoms appear three weeks later, they are simple, not quite white and they come together with foliage which is a bit yellow green. Not that I really mind, but I would like to know what species it could be. Some experts around?