The hard recoil spring is the spring on the recoil rod.
The only parts to which this broken piece might resemble to me would be parts 77, 83, or possibly 74.
http://www.wgcshop.com/gun-folder/ksc-g17-install.htm
Maybe you could obtain close-ups of the hammer assembly? A better look at where this part may have broken off from might be good. (I'm guessing it'f from part 83, the hammer sear.