Doug, discolored oxidation on unplated silver and unplated copper is often visible, but contaminants on gold plate that appear transparent on the connector(or hidden inside RCA and XLR panel plugs) become obvious with transfer to a swab. Whatever contact cleaner or tarnish remover you try, make multiple applications until a swab comes out clean. When you’re done you’ll have a pile of surprisingly dirty swabs.
BTW anyone who doesn't periodically clean unplated tube sockets and the fifty year old pins on vintage tubes should switch to SS gear.
That’s everything I know or care to know about it.