This firing pin retaining cup is made of white polymer.
It's a factory original part from Smith & Wesson.
Full Sized & Compact versions of the M&P M2.0 / 9 / 40 / 45 models as well as the M&P Shield 45 M2.0 models of pistols from Smith & Wesson chambered in 9mm, .40 S&W, & .45 ACP