Skip the washer—these metric stainless steel nuts have a flange that does the same job, spreading pressure across its surface for a strong hold. They're also called collar nuts. By leaving out the washer, they save assembly time, which adds up on parts that have multiple bolts. On the other hand, they may not fit into tight spots since they have a wider profile than standard nuts. All are 18-8 stainless steel, which is nearly as strong as steel and stands up to outdoor, washdown, and other corrosive environments without rusting.
These flange nuts meet ISO or DIN dimensional standards that confirm their compatibility with other parts and tools. ISO 4161 is functionally equivalent to DIN 6923, which is no longer active but still a common reference for designers and makers.