Comment by internet_points

https://en.wikipedia.org/wiki/APL_(programming_language)#Har...

you would add a special "typeball" into your IBM Selectric Typewriter. Some pics:

https://www.duxburysystems.org/downloads/library/texas/apple...

https://pierce.smugmug.com/Misc/APL-Typeball/i-pjq6hWC