Others will answer this better.
but Yes the discolor is Color case hardining.
the rest of the answers are available at the FAQ section.
But the firing pin hole is what makes one model shotgun, rifle, or rimfire.
Some rounds like the large primer pistol rounds (44 & 357) can be shot on the shotgun frames
shooting a 30-30 on that frame will cause some problems.
and the rimfire is self evident as it is off center and will not hit the primer on the center fire rounds.