I have opened case back on Sinn 556 (had to send it for engraving) and noticed that there is a fluted metal ring (pls see photo below) sitting between case back and the case. It seems there is no dedicated rail to place it in when reassembling, so I turn to you for guidance :). Should I place it on the inner side of the case back (that's the only place where it fits +/-) and screw it (case back) in while holding watch horizontally dial facing up? Because if I turn watch face down (e.g. on the table) and place this ring on top of the movement , it does not stay in place. I am sure it is there for a purpose, just can't figure it out .
Thanks a lot as always!