Holes in Sandstone

Walking the beach at low tide and depending on how the sand has shifted often spot something that I hadn't noticed before. Yesterday, quite near to the rocky outcrop close to Lower Largo Harbour, noticed these flat red sandstone rocks pitted with holes. They are likely to have been made by a rock boring mollusc, such as a common piddock (Pholas dactylus) or more likely, since they are quite small, a wrinkled rock borer (Hiatella Arctica).

Also came across this small (10cm in length) piece of sandstone with some ring markings. Not sure what these have been made by - whether natural or man-made?