Re: Argentine Navy Colt
The dark part is due to front of the slide having been heat treated to harden it. Through WWII, Colt slides were not hardened except in specific areas (the front, and around the slide stop cutout). Those guns were commercial models and had the standard commercial blue finish of the time; yours was Parkerized by the importer. For some reason I don't know, not being a metallurgist, that heat treatment shows through Parkerizing even though it did not show through the original bluing.
That gun should have the Swartz safety, a firing pin block released by the grip safety, like that used on some M1911A1 clones today (but not by Colt, whose Series 80 firing pin block works differently).