Some holsters and slings really produce a lot of the verdigris while others produce little. I suppose it has to do with the amount of tannic acid left in the leather from the tanning process, but I have holsters that the verdigris has pushed the washer off the rivet. Model 1907 rifle slings are another source of verdigris as brass was used up until WWII, and the verdigris will grow around the brass frogs or hooks and where the D rings are sewn in.