EPROM programmer voltage/signal switch
When PB7 of the 8255 is asserted the 2N2222 sinks the LM317 reference pin (terminal A) to B (-1.4V). The LM317 always outputs whatever the reference voltage is plus 1.25V, so bringing the reference to -1.4V gives us nearly 0V on the output. In this way ZIF pin 1 (A15) is driven low. It could be a little negative but that is tolerable to most EPROMs.

See Bruce Abbott's comment about the function of the 2N2907 (thanks!)

You are correct in that when driving A15 low you'd want to drive the 7407 inputs high so they don't contribute to the voltage at the reference pin.