https://www.superiorwasher.com/washers/9089/Stainless-Steel-Special-Washers.html
This video provides an easy-to-understand description of a special type of washer called stainless steel special washers which are predominately required in military applications. Their dual function as a fastener and a spacer is provided by these specialized fastening hardware. Stainless steel special washers are made from time-tested and reliable stainless steel grades including the 300 and 400 series alloys. Applications that require specialized fasteners and spacers can count on stainless steel special washers for optimal performance.