I'm always tinkering and creating new Japanese salad dressing. This is one them I created 7-8 years ago and very tasty.
Umeboshi can be found in Japanese markets in the refrigerated section. It is often called pickled plums. However, ume is closer to small apricots. The size range from dime to 50 cent piece. Smaller dime sized umeboshi is usually very crunchy and should not be used for this recipe as there is not enough fruit and taste is lacking and briny.
Ume flavor is very, very popular in Japanese cooking. You can find ume confection, ume rice crackers, ume candy, ume ice cream, ume pork chops, ume drinks, ume in various salad dishes, and of course, ume dressing like this. Umeboshi is loved by Japanese as much as tea and rice.