OK, the chip is a genuine mil spec part, with a batch code and testing lot number. It can be replaced with the 74LS version with no problems, and simply add a 5V1 400mW zener to Vcc ( between pin 14 and 7 on the solder side is easiest) to give a regulated supply to it. That it ran for decades with the bad power is simply because the chips are old, and the processing gave a part that would not blow up at 12V, while those that would blow up at 12V were weeded out very early as field failures. The 7V rating was because some parts would blow up at 8V, not that they all would fail, just every so often a batch would not survive it.
I had a computer where the power supply did a smoke test and applied the unregulated 16V supply rail to the whole unit, and the only failure after it ran for a few hours was a PM5403 hex inverter with 30V open collector output, which showed how unhappy it was by delidding itself then blowing the lead wires off. All the other TTL survived, even the unobtanium Intersil IIL DAC, which was on the board which made up half of the ADC system, and even the Intersil MK4007 serial ram chips survived unharmed.