Andy, one of the points being missed here is that photograde gelatins of the 1920s and earlier were active gelatins and came in 3 or 4 grades which controlled speed and contrast - and caused emulsions to vary a lot depending on the diet of the cows.
Modern gelatins are specially purified to give constant speed and contrast, but this speed and contrast are quite different than the one the 12920s writer intended. Generally, modern gelatins make emulsions that are up to 5 stops slower than those of the 20s and have much lower contrast. And so, AJ12 is nominally a nice long scale emulsion with an ISO of 25 or so if made with old gelatins (or made using modern methods which I teach). Without active gelatins or without modern methods, you get an ISO 1 emulsion with short latitude.