@MossRC Reminds me of Hellcats Over the Pacific and F/A-18 Hornet, by Graphsim. When you launched the simulator it would tell you to open the manual to a certain page and find a word at a certain position on the page. For example page 87, seventh line, fifth word. It wouldn’t run unless you typed in the correct word. Not exactly unbreakable, but it meant you had to have a complete copy of the manual, not just the software.
@MossRC Ah, makes sense. I guess in the old days of limited storage and compute resources, they probably wouldn’t have wanted to waste too much on a full copy of the manual, which was not exactly a small book.