It's a Flintstone mailbox.
Actually, I think it's what it is: a stone with a hole in it, placed there for it's general mechanical purchase. Possibley an old broken piece of a millstone or some such, reborn to do duty around the homested. Good for hauling stuff up/or lowering down, the hill. A rope from the house to the stone, then down to the road would also come in handy during a freezing rain or ice storm.
Bluezman