Overview
First of all, we don't know exactly what this is. And googling "Strapon dagger..." is a little scary on a work computer, but here's what I think.
What it is, is a blued steel double edged dagger, that is affixed to a metal plate, and sown into a leather arm/leg/elbow/head strap with buckles. It has .175" blade stock and .850" blade depth, 3.75" long blade. Some of the stitching in the leather has come out. Has a leather sheath that secures with two snaps, the tip of the sheath is missing a stitch or two. Otherwise this thing is in good condition for its age. Assuming it is old.
We suspect that this is a trench warefare item from somewhere in the WWI neighborhood. If anyone has any ideas or knowledge about this item, we would love to hear about it.
One thought is that is could be worn on the back of the elbow to jab attackers from behind.
All I know is it is intriguing and super neat.