“Brass” refers to a mixture of elements (mainly Copper and Zinc). The common mixture in the 1920s was different and included more lead.
Also, metals corrode over time. The older the metal, the more it will have corroded. When brass corrodes, it loses some zinc and can change colors. That obviously impacts the way it looks. Older brass is much more likely to have undergone substantial dezincification (the process where brass loses zinc atoms).
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