The Twin Horn Receiver displayed in the exhibition is the 15 Giga Hertz one. The instrument is an example of a Dicke Radiometer.
Radiation coming from the cosmos is reflected into the horns by the plane mirror in front of them. There is an angle of approximately 5 degrees between the axis of the two horns. The instrument measures the difference between the signal arriving from the two horns.
If radiation was uniform, there would be no difference between the two signals. However, a non-zero difference is a measure for a fluctuation if bachground radiation, or anisotropy.