Omega Speedmaster Mark II. Gold plated or Goldfilled?

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Your watch might have a stamping on it somewhere that says (for example) 20 M, for 20 microns. If it does, it is possibly gold plated. But then it could also be PVD (physical vapour deposition), or CVD (chemical vapour deposition). Three possibilities, and I know no way of telling one from the other(s).
 
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the 20M stamp should look like this, if it's still visible.... btw, dial and crystal are wrong for this Ref. number.