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In the early systems and I think that has changed the PNR used to be the hash of physical memory location of the booking.

Even back in 2002 most of the software used to deal with reservations was written in Assembler. I don't know if that has changed.

The six-character confirmation code you get when you book? That was the block address on disk of the start of your PNR.

Yes. The booking code is called the PNR. The passenger name record.