We still do similar things now, though for ostensibly different reasons. Inkjet and laser printers have long had various signatures they add to every printed page, barely noticeable to the naked eye, that can lead back to the specific printer used. The stated motivation is to prevent counterfeitting. Similarly, there is a pattern of "O" symbols called the EURion constellation that, if present in an image file, most commercial image editing software will refuse to print [2].
It's not surprising that politicians are trying these sorts of strategies with 3D printing, because they've already tried and used them often in the past.
[1]: https://en.wikipedia.org/wiki/The_Lives_of_Others
[2]: https://en.wikipedia.org/wiki/EURion_constellation#Counterfe...
- look up your local laws! usually you can photocopy money as long as it's shrunken / enlarged by a specific amount
- you can cover up everything but the EURion and see what it does to the rest of the copy
- you can cover up the EURion part of the bill and print out the rest
Again, particularly in the last point, be sure to look up local laws and set the printer scaling accordingly
Perhaps the CIA wanted people to use fax for secret communications, so they could use automated means to read the text instead of having people listen to hours of phone conversations?
Consumer fax calls were cleartext were they not?