Q: What's the difference between the "North America-Japan" and the China plugs?

I don't need to google the image, I have them both since I bought the Apple Travel kit. And, as I said, they look the same, apart from the holes.

According to the instruction manual, the holes are supposed to be on the US plug, while the China plug has no holes, but otherwise looks the same.

Usually there are no holes on US plugs, so this must be a way devised by Apple to tell apart the US plug from the China plug. But if otherwise they are the same, why would I want to tell them apart? So, there must be a difference, just not so easy to see. That's what I'm asking.