I have several signs that were displayed in a frame or were made in a way that they did not have any mounting holes. I also has a couple flange signs that I wanted to mount flat on the wall. Drilling non-factory holes was just not an option for me.
After a talk with my local old-school hardware store guy, I walked out with several sets of mirror hanging clips.
The clips mount behind the sign, mostly out of sight when finished. The upper clips are spring loaded. To install sign, put the sign into the hook of the upper clips and push up 1/4". Then swing the sign down into the hooks of the lower clips and the upper clips will follow the upper edge down to keep it secure. Measuring as the instructions specify is the key.
Doesn't take long. No need to drill the sign. Easy to remove sign, if needed.
Upper clips are spring loaded
Finished look of cast bronze sign plate with mirror mounting clips.