My frame has a third bottle mount under the front side of the downtube. Why, I have no idea, since it's fairly useless with the derailleur cables crossing and passing right over that area. In fact, I just discovered that the FD cable actually wraps over or around one of the protruding screw heads and rubs against it. This might be why my FD seems to go in and out of adjustment frequently!
I can't just remove the screws since that would leave holes in the tubes. So here is my idea: I'll cut off the shanks of the screws, thread them into the downtube with some strong loctite or epoxy, then grind down anything that protrudes.
Does this sound like a good way to do this, or is there an even easier way to plug up these holes smoothly? It's an aluminum frame, BTW.