Difference 312 and 512: via Paul Wolf of Tonelux
The similarities are that they use one 2520 op amp and one 2503 transformer each, which is the larger transformer. The differences are that of course, the 512B/C brings out all of the knobs and functions to the front panel, so X that, the differences are that the 312 has no coupling capacitor between the output transformer and the op amp, which means that if the op amp has any offset, it may cook the transformer. The 512C has a very large coupling cap, in parallel with a high frequency cap, in parallel with a resistor for transformer damping. The op amp has a servo to eliminate offset, resulting in two things, increased headroom and the elimination of popping when the phase switch is used.
The 512B and C were the next generation of the original 512 mic pre, which was the first modular mic pre and was used for mics, line amps, summing, and booster/distribution. Different models had meters, banana jacks, input selectors, and pots, in just about every combination, in the original consoles that API made in the early 70's. It is most similar to that instead of a 312 card, which has no features.