This document is for a Saturn 1B but I think it still applies.
The vertical bar of black rectangles are called "vertical motion targets".
Source APOLLO/SATURN CONSOLIDATED INSTRUMENTATION PLAN FOR AS-204/LM-1 pp 3-5 & 3-6
Special cameras were pointed at the targets.
4.4.2 FIRST MOTION AND VERTICAL MOTION OPTICAL COVERAGE. This information is provided by 16mm Milliken and 35mm Mitchell cameras. The two Milliken (DBM-4) cameras are located on holddown arms, 180 degrees apart, looking at first-motion targets near the base of the launch vehicle. They will operate at a frame rate of 400 frames per second (fps). A 35mm Mitchell camera will be sited at the 110-foot level of the umbilical tower, centered on the motion targets (Figure 3-3) located on the Position II (Fin 3) centerline of the S-IB/S-IVB interstage. The cameras will provide pictures showing first motion and the first 3 to 5 meters of the vertical movement of the space vehicle.
(emphasis mine)