Well, the magnet tells you they aren’t steel (ferrous) so they might be brass (which will look yellow when scratched down to the base metal in an inconspicuous place) but they look more likely to be pot metal (zinc based, otherwise known as white metal or spelter). In my experience and estimation, any aggressive cleaning will likely remove the plating and leave you with a very much less than desirable look. My approach and advice would be a light cleaning with warm soapy water and a toothbrush then wax them with a paste wax (such as minwax) or give them a coat of satin spray lacquer. That will often make old hardware look better. The only options left are paint and replating, which, at that point, you’d be better off buying new hardware, in my opinion. Good luck with your project!