A Wickson Hybrid
The plum here shown duplicates in many ways the appearance of the Wickson parent, although it shows perhaps a greater tendency to assume the form of the Kelsey. It is not as palatable as it looks; it is shown as illustrating how a crossbred fruit may simulate the appearance of its most valuable parent, and yet have no commercial value.
This image is from: Luther Burbank: his methods and discoveries and their practical application. Volume 5 Chapter 2