340 Heads

looking for some info..I have a 1970 340 with J heads.. both have J cast on outside and 3418915 cast top and bottom. both heads have stamp 10871 on edge by front lower head bolt. left head has 360 cast on top by valve spring and other has 340 cast in same spot and also has U cast under intake runner and J on outside. It seems I have one 340 and one 360 but odd that they have same date stamp.