In 1985, I took an evening class in APL at the local IBM as part of a high school Explorer Post (which was really cool, these guys giving up their time to teach us programming). Other than this, I am woefully inexperienced in computing on IBM machines.
Now, this was 1985, so the monitors and keyboards were already set up for us to just work without overstriking. HOWEVER, I proved to have some small talent at APL programming, so the guy took me aside and said he wanted to get me going on APL2, and gave me a complete set of manuals. ("We only have two copies of these. One we're keeping, and one is now yours." Like I said, really cool guys.)
The keyboards were NOT set up for the new APL2 functions and operators and so a form of "overstriking" was needed by typing two symbols with an underscore in between. On the older monitors, that's how it was rendered, as three characters and you just had to remember that this represented a single symbol.
But they had a handful of newer monitors that when any of these APL2 specific characters were entered with "overstriking", the symbols would sort of gloomph together and be rendered as a single-character APL2 function or operator despite there not being a key for it on the keyboard.
Does anyone know what that monitor might have been? I assume this was handled as a form of hi-res graphics, since I doubt these symbols were in the monitor's character set. A pittance of Google research has said that the 3179G was available around that time, and it had hi-res graphics. But I honestly have no memory of the model I used to program APL2 in those days.