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Re:Letterlocking by codazoda on Tuesday March 23, 2021 @05:24PM (#26560585)
Apparently these are called a "paper clinch".
Re:Letterlocking by Animats on Tuesday March 23, 2021 @10:13PM (#26562780)
There are crimp-type devices, and there are ones that cut a slot and tab. The slot and tab devices were patented in 1910. The main one was the "Bump Paper Fastener", which shows up on eBay now and then. So that's automatic letter-locking.

[1] https://pdfpiw.uspto.gov/.piw?docid=01065904&