John,
The centre piece is a composite lead-antimony and brass construction, patented by a Mr Cuyler Findlay in December 1915, as one solution to easing the production of Mills centre pieces - machining the early aluminium blank castings was problematic.
Some other photos of this very centre piece (it's actually a very well travelled centre piece...) show the construction of the lead alloy cast directly around the brass striker and detonator sleeves. Also attached is the relevant page from the patent - see Figure 3.
Tom.