"They first appeared as tape transport symbols on reel-to-reel tape decks during the mid-1960s." See https://gizmodo.com/the-secret-histories-of-those-ing-comp

"They first appeared as tape transport symbols on reel-to-reel tape decks during the mid-1960s." See https://gizmodo.com/the-secret-histories-of-those-ing-computer-symbols-5612630 https://gizmodo.com/the-secret-histories-of-those-ing-comput..., which is linked to from this article which you may also find interesting: https://medium.com/@mwichary/the-ghost-tech-in-todays-language-c3becc9ae30b https://medium.com/@mwichary/the-ghost-tech-in-todays-langua...