These are black EPDM rubber silencing washers that are commonly used with TLMAB mounting posts, chicago screws, slotted posts and other 3/16" diameter sheath and holster mounting assemblies. They are used to cushion and silence the contact points between the holster and mounting attachments. We've tested many brands of these washers and these are the top-of-the-line. They are used and endorsed by professional holster and sheath makers around the world. EPDM synthetic rubber is a bit more rigid than neoprene and allows less flex between the attachment and the holster/sheath. This is preferred by some custom holster and sheath makers.