They have been around since at least the Civil War era used as curtain and coat weights. How to tell if yours is old? My theory is to look for a mold injection mark on a modern one because they are still made today.
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Because lead is soft, I assumed it was flattened on the edges after manufacturing. Here are some examples of a modern curtain weight. They have numberes on them and a mold injection mark should be visible on the backside. Drapery Supply Products