Thanks. It is a Thos Priek caseback: http://www.customwatchconcepts.com/index.php?cPath=3&XTCsid=3l42srto5gusuifffqlfujfi52 As I found out when I took it for service, Omega don't approve of them & will not guarantee repair work if one of these is on the watch - so I put the original back on for the service. The reason I was given was that the caseback is not regarded as a perfect fit and will allow dust in if not fully tightened to a quarter turn or more. The answer obviously is to ensure it is fully tight and protect the watch in the way you would normally protect a vintage watch.