Hi Yangwater and Bo!
Mr.Muramoto produced umeboshi using apricots and distributed them through various outlets including his store in San Francisco in the 1980s.
See his recipe in "Healing Ourselves"[out-of-print but often found through Bookfinder.com].
Dr. Ushio's "Ume Plum Secrets" (
http://www.celtic-seasalt.com/happresbook.html ) and (
http://www.gomf.macrobiotic.net/Books_Health.htm ) is an excellent book which not only teaches why the umeboshi works, and the many benefits of regular umeboshi consumption, and of course how to make umeboshi but also ume plum wine.
When available one can buy Junsei and Kazuko Yamasaki's umeboshi plums made with celtic salt (
http://www.celtic-seasalt.com/umeboshiplums.html ) as featured in the above book.
And if that's not enough, Michio Kushi with Doctor Marc Van Cauweberghe, M.D. have produced an excellent book: "Macrobiotic Home Remedies" (
http://www.gomf.macrobiotic.net/Books_Health.htm ) where the production, purpose of making, the scientific explanation of the effects of, the alkalizing effects of, the antiseptic and antibiotic potency of, and the general physiological effects of, as well as the various home remedies using umeboshi are discussed extensively.
Please enjoy.
Thank you, very much.
Bruce Paine