Hiya,
There is only the one screw (visible on the top) that needs to be removed. The internal threaded bar you mention is just to hold the top on straight :-)
A good bit of wriggling will remove the cover and expose an original bakelite terminal block where you can easily replace the wires. It's worth noting that there is no strain relief mechanism inside the iron, so make sure the wire is terminated well or you'l potentially expose the live wires if you push the iron too far in use! :-o
Chat soon.
John Greenwood
November 2008