We used them to polish armatures on big electric motors, when I was apprentice.
Ran the motors & held the fiberglass brush against the armature as it went round!! (never lost an apprentice ) I shudder to think of what to put in a method statement/risk assessment
Still got one somewhere, have 2 propelling pencil types I use for cleaning brass contacts you can buy from CPC
I tried them out on various materials and simply couldn't make a difference to any of them - even aluminium. Perhaps the bristles are poking out too far I dunno. I'll sling them in the bin.
What are you trying to do with them ? Never had a problem with cleaning tarnished brass, copper, would have thought alloy was same. @Shox Dr making the engine turning swirl pattern, there were different grades if can I remember that far back, they certainly cleaned polished removed minor imperfections on the motors commutator https://www.cousinsuk.com/product/scratch-brush-roped
Before you do, try trimming the bristles off level with the paper wrap. Then peel some away so that you're using fresh f/glass. Might resurrect them, and find them very useful as the lads have said.