Likewise, I had never heard of them outside SC. From my readings, it appears they were dumped in the Congaree river in Columbia when the Yankees arrived. It was assumed they were awaiting sprue removal. It wasn't until a few years ago they showed up in excavations around Columbia, indicating they had been sent to the field with the untrimmed sprues. Prior to that they were only found in the river.
Interestingly, I read that the ones fished out of the river are always darkened because of a creosote plant dumping upriver.
Wes