edit: more photos of a model of Mariner 2 here at Historic Spacecraft's Mariner_2.
The images and quote are from the book Space Probes and Planetary Exploration William R. Corliss (written under the sponsorship of the National Aeronautics and Space Administration), Princeton, NJ: Van Nostrand, 1965.
How exactly does this ridged surface "diffuse infrared radiation without defocusing the microwaves"? These are millimeter scale, with increasing pitch at larger radius, so I'm guessing that they contribute to the focusing of the microwaves.
I'd really prefer a supporting link that explains how these ridges work to maintain focus simultaneously on two different wavelengths I can guess how (19 and 13.5 mm are close to the ratio 3/2), but an official explanation would be much better!
A solid, aluminum, parabolic dish 48.5 cm in diameter was used on Mariner 2 (Fig. 14-1). Its field of view was 2.2° and 2.5° in half angle for the 13.5- and 19-mm channels, respectively. The ridged surface seen in the photograph diffuses infrared radiation without defocusing the microwaves. Infrared heating of the radiometer is minimized in this way. Ten hours prior to the Venus encounter, the whole dish was driven by a scanning motor in a search mode at 1°/sec. When the limb of the planet was acquired, the scan rate was reduced to 0.1°/sec.
The first figure is a check that the radii do increase exactly as the square root of the ring number, which is what one might expect for a diffractive surface. The line is just 55 * sqrt(r) where r is in pixels.