Alan,
The earth pin on an AS/NZ plug is always longer than the other two pins, so that it "makes" first and "breaks" last.
The overlap at the back is for the switch mechanism of the socket, there is a plastic toggle that sits on top of the strip of brass, it is held in place by the front plate.
Them reddish spots are actually part of the "plastic" moulding, this was from a time when plastic was first being used to make electrical accessories, as opposed to the bakelite that was used in earlier fittings.
This plastic isn't unlike bakelite, as in, if you aren't careful and drop a socket like this on a bare concrete floor, you can almost put money on the fact of it breaking into at least 3 pieces.
