NOS Exhaust Manifold Heat Control Valve Counterweight Clamping Screw Lock. This Lock is installed onto the Clamping Screw of the Counterweight before you put on the Nut. You then bend the two Tabs over against the Nut once the Nut is tightened to prevent it from coming loose. It was used since 1953-54 on most Exhaust Manifolds. For Example, on all 1960-69 and (70 & Up Export Only) Slant Six Engines, all Poly Engines up to and including 1964, all Small Block up to and including 1969-Except the 340 Engines, and all Big Block Engines up to and including 1966. Was never used on any of the 426 Hemi’s. OEM Part # 1409594. Order as Required. We have a Limited Supply of these. Contact us, if you need us to check for correct fitment to your needs.