Bob, it might be simple in it's elegant visual impression, but certainly not it it's execution. The attachment method is very impressive as others have noted, as are the top finials.
I'd be interested in a bit more detail on the slot & back screw approach. I'm guessing a "tab" of PVC that is slightly below center of the sign, cut with a hook almost, to fit into a slot on those mounting bracket pieces & lock down into being centered to it?
Besides wanting to be inspired by any good ideas I can wrap my head around, I just struggled with a similar sign, so I wanted to find an improved method now.
Mine was a simple 2 sided non dimensional panel that I agreed to a 48 hour rush completion, because the largest golf tournament in Maui was starting this past weekend & this new restaurant was right in the middle of that territory, struggling to get signage approval in time. I designed & printed the layouts, routed the PVC panel & the posts the first day, primed & painted the posts that night, laminated, applied & assembled it the next day... but I only used PVC to save me paint time over the MDO option I usually would have used. I routed 1" deep, 1/2' wide grooves into 4x4 posts & had corresponding tabs in my panel. The realization I encountered half way through was that I am relying on the ground to hold this thing together. Because it was owner installed, I screwed temporary bracing boards across the whole deal to ensure that they didn't snap off a tab, or lose the parallel alignment of the posts. The slot & groove joint was very tight & assembled with a rubber mallet, but the freshly purchased 4x4's were still drying too...
Without too much more trouble, I could have used a solution like yours had I thought of it, or learned it here... maybe next time.