This document is for a Saturn 1B but I think it still applies. See below for the Saturn V info.
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.
Once you know what the document is called...here is the same info for a Saturn V (Apollo 15).
One camera at the 160-foot level of the umbilical tower, positioned to include a fixed reference target in the field of view will provide coverage during the first 5 to 7 meters of vehicle ascent. The frame rate is 96 frames per second.
Page 4-4 of APOLLO/SATURN V CONSOLIDATED INSTRUMENTATION PLAN FOR AS-510 (APOLLO 15)
I managed a quick visit to the JSC Saturn V. Here is a personal photo of the targets on its 2nd stage. Obviously it's been repainted over the years but I think they tried to keep it historically accurate.