While it is true that you can build logic gates using ... NAND gate) and then the logic was all in how you wired them together. And in this context, the SPDT relay was used as a two input ...

With the two diodes reversed and a 910 Ohm resistor removed, a NOR gate is created. The next step was to build a S-R latch using the NAND gates and inverters, which holds some basic memory.