Mysterious. That thrust washer has been in that position forever-- see the oil drain hole "shadows". The damage doesn't look recent or like an impact or gouge-- almost like a manufacturing defect. This washer may be reusable, or to be safe, order one the same thickness and another the next thickness smaller.
I be puzzled.
'73 R60/5 Toaster
Luddite. Not just a philosophy, a way of life...