Hi Sas,
The story, at least for Bisirri 3, ( the only one I have), came to me years ago from Ricci ( jazz bass), who doesn't post any longer. Mr. Bisirri originally had two varieties, light and dark. Mr. Bisirri supposedly had a large greenhouse where he stored his trees over the winter. However, Mr. Bisirri would also let some neighbors and friends store their trees in the greenhouse as well. Ricci was helping Mr. Bisirri along with Barry in the spring, when they noticed another tree in the greenhouse that wasn't labeled like the other two (light and dark). Later, Ricci noticed it was different in terms of fruit flavor and size. There is actually an old thread, I believe, that mentions this extra tree, eventually named Bisirri #3. This is the story I can best recall, but could be wrong about some specifics, it has been about 5-6 years.