Two of my bikes have a little knob thing on the inside of the rear triangle on only the side with gears and derailleur. It's about three inches up from the rear "corner" or the triangle. One of my other bikes has a tab thing that has a threaded hole on the gear side only. It is a couple inches up from the rear "corner" of the rear triangle as well. The tab extends forward. Can someone please tell me what these are for. Thanks.