This, or...
...A plate was made up with two nutserts in it and held on the inside of the outer bed wall or fender while bolts were passed through the antenna bracket and threaded into two plates per mount...
This. I personally prefer this method, especially if you're adding an antenna that may get hit by things, or is heavy. Grabbing two pieces of sheet steel with a spacer and load plate on the inside squeezing the spacer together in the body panel space, will give it some structure. Using only rivnuts sounds like a good way to pull the antenna off the body panel if it strikes something (tree, bush, bicyclist...
)
I saw a thread about those plates. I don't disagree but those holes appear to be in the correct place on the passenger side, just got ripped out maybe...
I don't think those holes were ever threaded or had nutserts in them from the factory, maybe a depot or field repair?
If you can't shine a flashlight into the hole and see light from underneath the truck in the body panel space, then it's probably a Rivnut - if so and there is not thread, that probably means some yahoo forgot lefty-loosy, righty-tighty and stripped them out (you'll have to drill out the old rivnut and install a new one if that is the case). If you can't shine through, it's possible the backing plate method was used to mount the antenna bracket.
...there appear to be 3 sets of '4 bolt pattern' holes (driver cab, driver tailgate, passenger cab) and the passenger tailgate just has one hole..maybe for a tie-down clip?
Tie downs I believe were two holes. One thing to remember, these vehicles have been reconfigured over the years, both by the military and after-owners (if applicable) - and with the military they often had to improvise due to time and budget constraints. Technology and usage changes constantly, they may have had an antenna in the rear and then moved it to the front (there is a model year effect on this too IIRC). For antennas that
were not multiband like the military uses now, they can't co-locate an antenna, but rather have to move it a sufficient distance away from one another so they don't interfere.
...we are trying to restore the vehicle as close to how it was originally so we don't want to add any guards or antenna brackets that weren't there originally...
Based on my above comment, I'd ask if you actually know what you are trying to restore to? If you are the first owner of the truck after the military, and were trying to go to 100% military authentic, I could argue, don't touch it - it's already authentic as-is!!