The missing bolt threads into a missing "U nut". Surprisingly, the first auto parts store I visited had these available. I just had to match a bolt to it, which came from my lifelong collection of loose bolts stored in a mayonaise jar. They often come in handy. Of course, no two bolt heads are identical, each one being a different size, both English and Metric measurements. This must be a rule somewhere with the Society of Automotive Engineers.