Dgroff, very nice! Great acquisition.
Blowing up your photos, I still cannot offer an opinion about whether or not it has been fired (post factory). On of the first places wear shows on Colt DA revolvers is the ring-like circular losses of bluing around the ejector rod, caused by pushing on the ejector rod, which in turn wears. If you see any such bluing loss, you can be pretty sure that the gun has been fired post-factory.
Fired or not, you have an excellent, extremely high condition Diamondback. Congratulations.