It's somewhat difficult to tell how much wear there is, yet this appears as though the base compound being uncovered. Nothing to stress over, then again, actually you are exceptionally close to expecting to supplant that tire. ***Link Removed***
Foundation: When a tire is made, the track elastic is expelled in a section, slice to length, at that point folded over the uncured tire and the parts of the bargains piece are grafted together to frame a smooth, consistent looking track. (Note: The track join is cut at about a 45° point)