It's the Sight seal for an M1 rifle. These are hard to find, they were destroyed every time the front sight was adjusted, when the rifles were refurbished at an arsenal or in the field they were destroyed, and the stocks of them were quite few in number. I picked this up for a 1941 M1 that I plan on restoring. I doubt if I'll put the seal on the screw, but it's nice to have it. I paid 27 dollars for it and have seen them go over 50 dollars on flea bay, I'd guess the value will go up because of the rarity. Anyone else collect M1 parts?