Hi David
You mean a groove around the circumference inside the bore of the hub, halfway along? I don’t have a groove in mine but, in general the hub should only contact the stub axle shaft at the inner and outer ends of the taper – not along the whole length. Since the taper on the axle shaft is a linear taper, the hub is the part that has to have a more complex profile. I guess the groove is something to do with the machining process that enables the areas at the two ends to be machined and checked without interference from the relief in the middle bit. Or something like that.
Chris