1921 P Morgan Dollar Edge Damage.

Typically, the year meant something to the person making jewelry, nothing to do with any rarity.

On the weight issue, I thought some math might be fun. The coin weights 26.36g instead of 26.73g nominal weight. So it is 1.4% light. A Morgan $1 is 38.1mm in diameter and 2.40mm thick. Volume = pi r^2 * h, so the volume of an idealized Morgan $1 is 2736 cubic mm. Therefore if this coin has lost 1.4% of its volume, it's missing 38 cubic mm. The ground-out channel in the rim appears to be about 1mm wide (since the overall thickness is 2.40mm, that seems reasonable). A 1mm height makes the math simpler. It's basically calculating how much smaller a 1mm tall radius 19.05mm cylinder would have to be in order for the volume to be reduced by 38 cubic mm. The answer is 0.32mm. So a 1mm wide channel one-third of a mm deep easily accounts for the weight loss.