Regarding what Desk Jockey wrote about the Stress and Fatigue, probably links with the fairly major mods that needed to be done between section 41 and section 43. (see note below).
I heard that this was as a result of the Lockerbie incident. Some boeing guys I expect realised that the whole cockpit section had separated from the fuselage and investigated.
The series of Mods were created as SB's to strengthen and improve the fit between the two sections.
Thats what I heard.
Section 41 is the nose and flight deck and section 43 is the next part of the fuse (I dont have any accesse to 747 info any more and it was a while back so memory working here!)
Hay Ewe - pass another rivit