This has "W F Co" stamped on it. Could be Well Fargo, as it was found at an old stage stop. It probably is not actually a washer. Common hardware seldom has the mfr ID on it.
That is a jamb or locking washer. Larger end fits over a pin with groove milled in it, or a flat head and it is driven sideways to lock or jam on the shaft of the pin. At least it looks like ones I have used in industry.