A key to determining what it was for is the thickness and angles - and this is not visible from the images.
You say flat, it is completely flat? Does it slope to one
side or another? Does it have central ridge? Sharper edges?
It could be a piece of scarp with an interesting shape, javelin
head, spear
head tip or sword blade tip.
But spear or javelin would require that it have a slight diamond-shaped profile. Sword a slightly rounded or lenticular profile. If truly flat then I doubt it is from a
weapons.
SC