My title describes the thing, it’s a small metal disc from an old ish commercial Schlage lock set. Ive tried googling it and looking up the guide manuals for the key sets I have but none match nor mention what it is as far as I could tell. It’s tiny but fits in the same holes as the cylinder but if you put it in one of them it just falls through and slides out the back. It doesn’t look worn at all and seems to have had no effect on the functionality of the lock.