I don't have any Dianas with the Original logo, or the FOREIGN marking, but only US imports with Diana, RWS, Hy-Score, or Peerless logos.
Several of my guns are marked MADE IN WEST GERMANY, and others only MADE IN GERMANY. The ones with WEST included are definitely older. The oldest gun I have
without the WEST is a model 5 pistol with wood grip/frame, which disappeared from catalogs by 1965, according to Mr. Griffiths's outstanding new book. I have several others with MADE IN GERMANY that were definitely made decades before 1990.
So...if (and it's a huge "IF!") the date for changing to the MADE IN GERMANY mark on UK imports, corresponds to that for the US, it considerably pre-dates the actual re-unification of Germany.
Why that might be...I have no idea!