[Adger Smith]

X was put on cores that were used up and out of spec in some manner.
When they ran short on cores they might make a run with cores that were marked X for out of spec. Most X castings were done in core boxes that were out of spec with worn runner molds. That made the runners larger than spec when they were poured. There are some castings that have X's in the lower core box, water jacket passages, and no X up on top with the casting number. Only half of the core was out of spec in those cases. That is the information I got from a person that was supposed to know, a long time ago.
Most X castings I have had come through my shop were anywhere from 3 to 10 CC larger on the runner. Most of the time it was in the bowl area on the short side or by the guide. There were some with funny shaped Ex.