FWIW I had long thought of these as freeze plugs until some automotive machinist told me that they served as core plugs so sand could be gotten out of the casting at the factory. I have seen too many of these blocks that froze and cracked without moving the plugs. In my youth I worked in a welding shop where the boss welded and or brazed many cracked blocks. He had a big oven to put the blocks in before welding.
