This is a part of an Agilent signal generator output attenuator. These are 4 or 5 of these stages with the resistor setup as shown, different values at each stage to set different attenuation levels and these can be combined using relays to create several combinations. For example one stage is -2dB, another stage is -4dB but they can be both inserted inline creating (surprise) -6dB attenuation.
What I don't really get is the resistor arrangement. What's up with the values and why the network instead of 3 simple resistors?