Seamaster 166.091 Tested 200M inscription

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Hi friends. Any insight on this “tested 200M” inscription on case back of my SM 166.091 is appreciated. Checking this out on my own, found that most SM 166.091 do not have this inscription on their backs and it intrigues me. Watch is mine, bought new in the 70’s, and the case back is original. Haven’t found the answer yet. Perhaps somebody here can throw some light. Thx in advance
 
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During my long catch for one of these I observed that some have a caseback without inscription.
In some case it was hard to say if it came like that fron the factory, if it was canceled by daily use or by polishing.
At the I was lucky to find one with the inscription.
 
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Thanx so much, it was my assumption too that possibly inscription may have been polished off. However my first stop to check was “the net” and most but all 166.091’s featured lacked the “Tested 200 M” inscription. Somebody (watchsmith by profession) has suggested that manufacturers don’t actually test all pieces in a batch, but take representing samples of each production batch, respectively “signing” them. To be honest, haven’t found any further supporting confirmation to this procedure, so the quest is still open for me.
 
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Thanx so much, it was my assumption too that possibly inscription may have been polished off. However my first stop to check was “the net” and most but all 166.091’s featured lacked the “Tested 200 M” inscription. Somebody (watchsmith by profession) has suggested that manufacturers don’t actually test all pieces in a batch, but take representing samples of each production batch, respectively “signing” them. To be honest, haven’t found any further supporting confirmation to this procedure, so the quest is still open for me.

Could be something as simple as two batches of case backs...
 
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Could be something as simple as two batches of case backs...
As good an assumption, however… This is Switzerland we’re talking about and the Swiss are known to do everything for a purpose
 
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As good an assumption, however… This is Switzerland we’re talking about and the Swiss are known to do everything for a purpose

Yeah... that's what their public relations team would have you believe... but the fact of the matter is, the Swiss watch industry is full of odd decisions and strange choices.