Here at work, we are repairing the vacuum control circuitry for an
electron microscope made in 1979. It has 6 SN7400 chips. If I
remember, these are quad 2 input NAND gates. 2 of them have gone bad
because the heat sinking of the board is not very good, you can see
heat discoloration on the board.
Back in 1974, I was 18 and was experimenting with making computer
circuitry trying to make a computer cpu with a calculator chip and a
buffer made from a bunch of 7400 gates as memory flip flops. I never
got rid of all those old chips and still have several tubes of them
unused.
Now, 36 years later, I find a use for them.