That valve was only used on 69 and 70. The 70 also had a brass valve in the back on the frame by the rear wheel to hold pressure to the rear brakes. The distribution block you have on your frame does not look stock to me so you may not need the proportioning valve by the brake booster. If you use a distribution block for a 71 or 72 you do not need proportioning valve at all. Here's how the stock set up looks.
