The scar on Clark Luthor's arm is in the wrong place when he steps out the barn and the viewers realize that it is Clark Luthor. The scar is shown on his wrist but the scar is supposed to be on his forearm.
Clark Luthor (CL) uses his mirror box to transport from Earth-2 to Earth-1. When he arrives, he still has his mirror box with him because he leaves it for Clark Kent (CK) to find. It can be assumed that the box came with him because it was in his possession when he transported. He then comments about CK having destroyed his copy. CL next triggers the mirror box, sending CK to Earth-2. However, although CK was the only one holding the box, it remained in Earth-1.