The map now includes a nearly comprehensive listing of South Carolina Historical Markers. There are roughly two hundred markers (of 1,550+) that do not have pins. We will work to locate these on the map in the coming weeks and months. At least some of these unmarked markers have not yet been erected.
Updated October 30, 2014
South Carolina Historical Markers 
Can the public help contribute? I know a few in York County and Lancaster County that do not have pins. I’d be glad to help in the counties I travel in,
While there is not a mechanism to actually place pins on the map, any location information for markers not included would be greatly appreciated. You can send to efoley@scdah.state.sc.us. Thank you!