Saint-Benoît-sur-Loire to Bloise, through Orlean

I saw lots of these old stones, with numbers that incremented. I asked at a tourist office, the lady there said it had something to do with navigation and avoiding the sandbars, back when they actually had boat traffic on the Loire - or maybe on a canal to the side of the Loire. The D stands for "decendante" (decending). In other words, going downstream.