I thought my early plum might be a Satsuma, but now I'm changing my mind. There are lots of misidentified images of plum that turn up when I do a google search. Lon guessed Methley, and I think that is what I have. I found an old illustration of both Satsuma and Methley, and Methley is a good match. Here is a link to the illustrations. http://www.rosegathering.com/marieta/publications.htm
I have also created a photo album showing the mystery plum, if you want to help with the ID. Here is a link to those photos. http://tiny.cc/0msb8
-Mark Lee, Seattle, mystery plum and blueberries ripe now