Ahhh, thanks for that Nic, it really sheds some light on it. I am missing the zig-zag spring then.
Are they available to buy? (I read somewhere that Omega still hold parts for the 5100)

So, that's the springy ring sorted, should there be a screw by the 'D' of 'Adjusted' ??

: This is very common in Lemanias....especially the last series
: silverstone.
: the movement is held down with the retaining ring and then the
: zigzag spring ring ontop.
: the caseback then screws down ontop squeezing them.
: make sure the caseback thread is really clean first. add some
: angles to the zigzag spring as this can flaten. If your missing
: the spring try using thick rubber or nylon gaskets.
: good luck