Yes, headstud with the hex barrel nut riding along.
It's possible that you can drive with a broken stud indefinitely, don't think you need to rebuild right now. also, it's possible (but VERY unlikely) that it's a spare that the mechanic left floating around in there after replacing others.
Here's a good picture showing where the 12 headstuds are, I highlighted a structure that you can see in your picture so you know where to look (they are deep in there):
Get a flashlight and a mirror and start searching, account for all 12 of them. If you cannot see the hex barrel nut in the hole, then you have found your broken stud. If you can't find TWO nuts, then you have another broken stud as well
