well, you said you are going to wolfskill, and you know what? i could bet my vegas dollar no one there knows the answer either!
your work is trail-blazing, if not pioneering. The closest thing to expensive genetic studies!
for most of us(mere laymen, lol), the approach would be to compare the quality of fruit of this jigsaw nigra to standard nigra's. If quality is the same, then one could safely conclude that it may well be a hybrid, or perhaps a sport of nigra. May also be due to a nigra pollen/ovule) rendered aberrant by too much direct sunlight, etc. which produced viable but different nigra seedlings. Or perhaps even an alba x nigra hybrid!
moreover(possibly, but probably not likely), it may also be due to age of the specimen. In certain species, say, water lilies, their abaxial cells change in form and shape as well as the number/morphology of stomata as young plants just emerging from their tubers, compared to older plants, especially when their young leaves have yet to attain the length to reach the water surface/atmospheric air. Take note that mulbs exhibit heterophylly(fig-shapes to heart-shapes), and water lilies exhibit this too. Could be that younger mulberries, especially when grown from seed, have microscopic variations in the leaves they produce as they age. Certain mulbs(paks, and nigras) seem to vaguely follow the stem patterns of jujus, bearing fruit on horizontal growth and none on structural upright growth, and may need more than one season to attain readiness to fruit. One could consider the possibility that the leaves borne on such branches may have variances as well, depending on stage of stem development.