The original Sump Plug was cast Aluminium with wings and a hollow square drive -
as you can imagine over the years it had been cross threaded,
the wings were long gone and the hollow Square Drive was well worn.
At some timethe threads had picked up.
Time for a bit of a clean up and given the course casting it was decided
a dissimilar metal was call for as a replacement sump plug so as not to pick up again