These AN flat washers by Pro-Werks are specialty flat washers, made of non-hardened steel and are cadmium plated (gold in color). AN washers have a smaller outside diameter than flat washers that are commonly found in many industrial applications. 

Their primary purpose in the aircraft industry is to provide a smooth bearing surface under a hex nut allowing for index alignment of cotter pin holes or as shims to obtain the proper grip length needed on the fastener. Performance minded builders also prefer AN washers for applications where space availability is limited, and to save weight.