IBM Selectric Electric Typewriter left margin symbol indicator thingy on the little carriage position indicator plate between the keys and the platen of the typewriter. Most high schools had rows of these typewriters in a classroom to teach touch typing until at least the 90s when they were displaced by PC software. I don't think high schools teach typing anymore, would not work well on iPads.

I was there in the 80s, and I remember this character being used margin/tab displaying purposes in at least one word processor program. It seems an obvious use, IBM literally used that symbol for that purpose on their typewriters. Does anyone else remember the name of the word processor program?