Type 18-8 stainless steel neoprene bonded washers.
These washers are slightly convex (bulges outward) and are
cushioned with black neoprene for purposes of sealing.
Commonly used with self-drilling tek screws.
The neoprene portion is black with a temperature range of -45° to +250° F
and a durometer hardness of 65A-75A.