This tree was one of the first Luther Burbank Paradox Walnut hybrids from his working estate on Sonoma Mountain circa 1851.

'Luther Burbank began making controlled crosses between walnut species in the late 19th century after hearing about a “supposed natural European hybrid walnut.” He crossed Juglans hindsii (northern California black walnut) × J. regia (Persian walnut) and produced progeny that he named ‘Paradox’ because of its extremely fast growth and other “anomalies.” Burbank’s contributions to the walnut industry endure to this day, especially through the widespread use of seedling and clonal ‘Paradox’ walnut rootstocks.'
~ American Society for Horticultural Science

x paradox = cross gender of English Walnut and California Black Walnut
Paradox Walnut (Juglans x paradox)