We don’t know what variety of walnut we have at the farm, but our guess is English walnut. The old hulls on the ground don’t look like black walnut shells.
The tree must have started to split at one point in the past, because the tree has grown around a chunk of steel all-thread rod in the crotch between the two main trunks. The ends of the all-thread rod probably have big washers and nuts on them, but the ends are no longer visible.
We have plenty of walnut fruits growing on the tree!
Update 7/24/2013: The previous owner tells us the all-thread was installed in 1982 to stop the tree from splitting. Looks like it did the job!