KL6610 vs KM6610 Seamaster De Ville

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Anybody know the difference? KM6610 is not found in the on-line Omega vintage database.
 
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I have owned both of these--as far as I'm aware they are the same. I believe both are wearing US-made cases by Ross. I forget if they are gold plate or Solid 14K, but that can be determined by the stamp between the lugs.