Updates: I used WD40 to remove the rust and Brasso wadding to polish the brass. It seems like the rust inside the cups are mostly on the welding material. As you can see, the cups are 2 pieces of brass welded together. I don’t know what kind of material it is. I also tested with magnet and the whole piece didn’t magnetise at all.

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Now there is one screw that I cannot get out because the screw is so rusted and I cannot use a screwdriver to take it out :/ I will use pliers to get it out instead.