These knives were clipped inside the high length boots of German soldiers (Wehrmacht or Luftwaffe). This one has the mark Adler 6 (eagle 6). The knives were used for close quarter combat. Lengh of the blade is 6 inches (15 cm.).
I'm Dutch but live in the UK. My father took this knife from a dead German soldier at the end of the war in the east of Holland.
He used it as a pen knife and after his death it ended up at my attic for 40 years or so.