http://www.aa.com/i18n/aboutUs/where...rminal_LAX.jsp

Most likely 43X is either the international gate(s) at Terminal 4, or a "bus gate" where you park on tarmac and are bused to Term 4. AA has its own immigration/customs facility in 4, though hours in the past have been limited, the facility used to close after the second LHR flight arrived, e.g. about 5 pm. This was in an era when they had no long hauls to Asia, only LHR plus some flights to Mexico and Central America. Because of overcrowding at Tom Bradley Terminal, LA has some rather elaborate bus gates, which are "semi-permanent" structures with what appear to be jetways and stairs or ramps to the tarmac level, at which point pax board buses----a semi permanent set up and a bit nicer than just stairs to the tarmac.

PS a high school friend is an AA FA and worked the inaugural flight to PVG. She sent me an e-mail from PVG yesterday, but I wasn't aware that she worked the inaugural flight until I saw your thread (I wasn't paying attention to the start date). She lays over today and I believe works tomorrow's return flight. I was not able to ask her about any special ceremonies because when i replied to her I thought the route had opened a few weeks ago.