The original IBM PC (8086) had the letters IBM coded into a specific part of the ROM. When PC clones started being made, IBM (and probably other software companies) would look for those letters to verify the software was running on a genuine IBM PC.

Rather than just copy those characters into their ROMs, the clone companies would add "NOT IBM" to get around any copyright claim - with the letters "IBM" in the same memory location as the originals of course. I discovered this when hacking on an Elonex PC - if anyone remembers those.