Turns out it would be a missing #6 from the link above. I find it hard to believe you'd put it back together without that washer, but...

I haven't pulled it apart yet, but I'm trying to figure out what they might have missed, or what might be worn on the front strut mount. On the passenger side the plastic cap is missing and the threaded bolt extends far enough that the former owner damaged the hood by slamming it down over the strut...