Here is a quote from AP 3357 Operating Manual of Ground Controlled Approach Aids dated 1960:-
SLA 3-C (Precision Approach Radar)
CIRCULAR POLARIZATION
Circular polarization is a means of making an antenna less sensitive to responses from rain, while retaining sensitivity to other responses.
On this equipment each antenna (Azimuth and Elevation) is fitted with a "rain filter" (often called quarter-wave or circular polarization plates.) For operational flexibility the rain filters are hinged and provided with an electric drive motor so that they can be independently switched "in" or "out" of operation either locally of from the tower.
On the previous equipment, the SLA 3-B, the plates had to be manually inserted, which took circa 30 minutes. The SLA 3-C plates took less than a minute to motor in or out.