M8 x 25 Penny Washer A2-304 Stainless Steel

These are flat penny washers sometimes called a repair washer or even a mudguard washer.  These have a wider diameter denoted by the size listed. The washers are used to spead the load even more or to take ip the slack so when tightening the fasteners the nuts and bolts have a greater contact surface to assist in keeping the nut or bolt secure.  Washers also take the pressure from just a small space by spreading the load from the component side to prevent wear in the connection of the fastening.

These are A2 stainless steel fasteners which has a preventative measure from corrosion. Ideal for use in damp or wet conditions such as outdoors or in water ingressed manufacturing.  Also bear in mind using different materials can cause fusing through corrosion especially in damp conditions when mixed with aluminium or other none stainless steel metal materials.

More information is available from Central Fasteners regarding fastener details, but its always worth checking with a manufacturer's specifications or drawings if an alternative size, tensile strength or types of washers are used.