TK is a very standard term, see William Safire's usage in this 1996 NY Times article: https://www.nytimes.com/1996/10/06/magazine/of-hacks-and-tk....
replyMmm. This Q&A -- https://www.chicagomanualofstyle.org/qanda/data/faq/topics/M... -- suggests it's been kicking around as printing and journalism jargon since at least the 1980s, and I would expect probably earlier.
replyPaywall-free link:
reply