There are no city stamps or date letters on modern Norwegian silver. Early pieces are sometimes marked with the year, e.g. 1926, and/or the city such as Oslo, Christiania (the former name of Oslo until 1906), Bergen or Trondheim. The bigger companies started using 'Norway' and the content mark 'Sterling' after World War II. However, since this has been a question of company policy there are no rules for this. When the word Norway or Norge is not stated, the first clue to identification of Norwegian silver is the content mark that gives the quantity of pure silver in the piece. This usually is either 925 S (meaning 925/1000 pure silver which is equivalent to Sterling silver) or 830 S ( a quality often called 'coin silver' in the US) which means 830/1000 pure silver and is often used in the Nordic countries.
Best web-site Norwegian hallmarks: www.925-1000.com/Fnorway_marks.html