I bought a used cable machine that was missing a pin. This worked great for what I needed. The magnet keeps the pen in place.
Pros
Please ensure you order the correct pin diameter and length for your weight stack. When measuring, the pin should go through the stack, change shanks, and fit nicely into the other half of the plate. The pin must cover at least 75% of the depth of the stack. The distance between magnetic pin locks corresponds to the length.