The washers avoid screw marks around the mounting holes on the module's faceplate and in case of the grey-green Cwejman modules chipping paint.
The bag contains one hundred black nylon washers with 3.2mm inner diameter, suitable for M3 screws.
Small hint: you don't have to apply force when screwing the modules into the frame... at a certain point the washer will collapse and the module will be scratched by the screw. Just screw it in until the module doesn't move, theat's enough. Take it easy :)
inner diameter: 3.2mm