I maybe wrong and I know that diagram says "brake warning light/proportioning valve" but I believe it is the same for drum brakes. The proportioning value is on the line that goes to the rear axle. It's located on the inner frame rail approximately a foot away from the picture you show. The one you show just detects an imbalance of pressure if you rupture a line and get a leak. I know at some point they did incorporate it together but I believe original equipment was still the kelsey-hayes calipers and bendix master in 1971.
