Don't know what bike this it going on... Assuming the 400 based on the screen part number.
But if those washers will be touching against any part of the screen then definitely nylon or rubber. NOT stainless. In fact, even though that diagram doesn't seem to show it, I'd put rubber or nylon washers between the screen and the metal bracket. Possibly a metal stainless washer directly under the head of a bolt with a nylon washer between that and the screen. Metal directly on the screen material can induce damage/fracture cracking in the screen material, especially from normal vibrations of the screen against any metal.
The washers that will be touching against metal to metal only can be stainless. I would suggest nylon insert lock nuts. That's always what I use to prevent becoming loose over time.
But if those washers will be touching against any part of the screen then definitely nylon or rubber. NOT stainless. In fact, even though that diagram doesn't seem to show it, I'd put rubber or nylon washers between the screen and the metal bracket. Possibly a metal stainless washer directly under the head of a bolt with a nylon washer between that and the screen. Metal directly on the screen material can induce damage/fracture cracking in the screen material, especially from normal vibrations of the screen against any metal.
The washers that will be touching against metal to metal only can be stainless. I would suggest nylon insert lock nuts. That's always what I use to prevent becoming loose over time.
