This is a Feldmeßkästchen 18, which was used by the Germans during WW2 to test and trouble shoot field telephone equipment. Perhaps you have one. Cool, huh? Did you ever try to use it in your Nachrichtentruppe impression? Did you ever wonder what the German writing inside the lid means? Read all about the Feldmeßkästchen 18 and its use in my book. The 225 page book is available for $44.99 though the Lulu website at:
WW2 German Field Telephone Equipment 