Essentially a Rangemaster with a switch selecting the input cap (Hi/Lo) and another switch (Ge/Si) selecting between a germanium or silicon transistor. The bias pot helps to fine tune the bias (imagine that lol) differences between germanium and silicon modes. Other transistors can be used other than what's specified. Something in the 100-120 hFE range for the germanium and something around 250 hFE for the silicon. Schematic here.