M3 = 3.2mm i/d x 7mm o/d x 0.5mm thickness
A2 stainless steel grade = 304 grade stainless steel.
This is the standard industrial grade of stainless steel. This has better corrosion resistance than standard steels like plain steel and galvanised steel. If you are using this type of product outdoors in a marine environment then you would be better off using A4 grade (SS316).
These are the washers that are most used around the world.