Both the water bottle bosses on the seat tube and down tube of my '97 Pinarello appear to be just holes in the tubing. But I've looked down the seat tube, and there are fittings (with threads) attached inside the tubing at each hole. So when you thread the water bottle cage bolt in, you're actually threading it into the internal fitting.
Last edited by well biked; 11-15-10 at 10:22 PM.