 Eventually these postal agreements were settled between Germany and other countries, including neutral countries such as Portugal, Spain, Switzerland, Sweden and Turkey, which had volunteers within the German forces.To preserve the secrecy of troop movements, each battalion was assigned a five digit code number called Feldpost number (FPN).By the end of 1939, letter prefixes "L" and "M" were attached in front of each FPN to units belonging to the Luftwaffe and Kriegsmarine.