It is a firing pin from another weapon and does not match the one from the Steyr. Obviously too, a firing pin on the outside of a gun is also quite useless
. So as far as the conceptual film world reality of the design of the gun, it is not a firing pin.
The "dish" is also not from the Steyr from what I have seen. The bolt as well, is a replacement and also likely fabricated for the prop.
Not sure if there was any real intention on the prop maker to create something with any real conceived function other than to just look cool, but part of the fun of the gun is trying to see if we can at least pretend to reverse engineer it into something at least partially believable as it would exist in that universe.
Andy