Germanium semiconductors aren't very common anymore, and in the case of the 1N4148 diodes in the x0x power supply, it doesn't matter. The main difference between germanium and silicon diodes are the forward voltage drop, being about 200-300mV for germanium and 600-700mV for silicon low-current diodes.
The diodes in the x0x PSU are providing a stable voltage that is adjustable via TM6. There are two in series, then a potentiometer (TM6) in series, then another two diodes in series. Basically, TM6 is sitting at a relatively unchanging voltage determined by the voltage drops of the diodes, and the position of the pot wiper is connected to the adjust pin of an adjustable voltage reference (IC22). As far as those diodes are concerned, all that is important is to have a stable voltage, which can be accomplished with any choice of diodes. This subcircuit provides the 5.333V supply.