Your question got me thinking. I had read Farber's books a few years ago, but didn't remember exactly how he achieved his look. Last night I looked back through "Farber Nudes" and "Natural Beauty." He describes several techniques for acheiveing the soft, grainy look of his photographs.
From "Farber Nudes"--
For grain, he states that he shot Agfa B/W 400 ASA, pushed 3 stops, processed in Rodinol; for golf ball size grain, he used "recording film" @1600, developed in Rodinol 1;25 for 12 minutes. Also, for one shot he used Agfa 100 pushed 3 stops in Rodinol.
From "Natural Beauty"--
A number of his photos were taken with Polopan 125, in 35mm format. Several of his grainy photographs were taken using an 8mm Minox camera. He states, "Using the Minox TLX, I was working with a miniature format that happens to be a fine-precision camera fitted with the best optics." p. 203, plate 72.
He also took several of the shots "with slight diffusion." I assume he used a diffusion filter or some cover over the lens like a stocking.
Hope that helps.