A problem that I was noticing with the PKZSE models was that the frame and slide were often a slightly different color. At least with the BKO models the finish usually matches. If it matters much, in early 1943 the various contractors (Remington Rand, Ithaca, and Union Switch & Signal) used a Du-Lite blue finish instead of Parkerizing, which essentially was a matte blue (aka black oxide) finish.