Feldpost number
Feldpost number (FPN) (German: Feldpostnummer) was an identification number for a German military unit during World War II used for military mail service which means when family members and friends wanted to send mail to the corresponding unit. It wasn't unusual for the same feldpost number to be used for different units throughout the war. As one unit would be destroyed, then the number might have been reallocated at a later date.
The Feldpost number was a five digit code number and 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. A breakdown by military units was added by attaching letter suffixes "A" through "E" at the end of each FPN. The letter A generally signified headquarters company, the others stood for line companies. The sequence of a Feldpost number did not necessarily mean that the location of the units were at the same area.