And once you've had some good reading there, try looking at this:
http://www.925-1000.com/a_christofle.htmlLooks you might have an offset version of a 1935-83 OC mark. The 18 is sometimes set clearly in a separate box below the maker mark. If I'm reading old thread info correctly, this would be a third tier ("cafe" quality) service. Not familiar with the Apollo mark...they sometimes have the Alfenide metal/factory mark in the same location (which may relate to the metal line here too). Metal Blanc is another you might see on other pieces for the base metal. Good Luck.