A little over three years ago I reported that I found a Winchester primer without a anvil. I found another one today. The primer on the left is normal. The one on the right is missing the anvil:
I searched through my log files for all the rounds I have reloaded with WSP (Winchester Small Pistol) primers. I have reloaded 62,719 of them and found two missing anvils. While rare it does seem like a more frequent event that I would have expected.