[AZKen]

Could be after thought by factory to add reinforce. Could be no access for rivet tools. Could be something else mounted/used that bolt that needed to be serviced. I've seen other situations like that on trucks. Maybe PO drilled it?
RE fix: Yes, many options. It's not an exact thing. Just provide a place/bolt/stud/hole to mount the shock in approximately the same angle and distance as old one. Be sure shock clears bed structure, etc. I can not advise with confidence until I see what the area and bracket looks like. Don't have that bracket on my 60 4x4.