This collection of examples of licensing practices is mainly based on Aymeric Mansoux’s PhD thesis, and specifically on chapter 5.1 (for if you’re curious to read it yourself!). It mentions different variations/formats of copyleft statements, which are transfered in small typographic images on this page.
Aymeric Mansoux, Sanbox Culture (2017) https://www.bleu255.com/~aymeric/dump/aymeric_mansoux-sandbox_culture_phd_thesis-2017.pdf
Aymeric Mansoux, My lawyer is an artist (2011 + 2014) https://www.hz-journal.org/n19/mansoux.html
MacKenzie Wark, From Mail Art to Net.art (2002) https://www.nettime.org/Lists-Archives/nettime-l-0210/msg00040.html
a Xerox Mark - Radical Software, Volume I, Number 3 (Spring 1971) http://www.radicalsoftware.org/e/volume1nr3.html
Georges Machiunas, Letter to Tomas Schmit (1964) + Ben Vautier, “TO CHANGE ART DESTROY EGO” (?) in Theories and Documents of Contemporary Art: A Sourcebook of Artists’ Writings (1996) by Kristine Stiles, Peter Selz https://books.google.nl/books?id=WXV-HlsUzdcC&pg=PA726&dq=GEORGE+MACIUNAS+-+Letter+Tomas+Schmit+(1964)&hl=en&sa=X&ved=0ahUKEwjE1oXm4cHmAhXsQEEAHUzkDhoQ6AEILzAB#v=onepage&q=GEORGE%20MACIUNAS%20-%20Letter%20Tomas%20Schmit%20(1964)&f=false
Ted Nelson - Transcopyright, part of the Xanadu project (1960, updated in 2016) http://xanadu.com/xuTco.html