Absalom
SWCA Member, Absent Comrade
Interesting, so the finish is like Parkerizing but not exactly due to trademarks?
The exact timeline has been subject to some debate over the years, especially since the terminology back then, and as a result in the history letters, is not consistent.
The general consensus emerging is that after switching from polished Carbonia blue to a "brush blue" around December 1941, the company adopted the black oxide process called Black Magic and patented by Hubbard-Hall with just sandblast prep around April and used this through the end of production in 1945; wartime documents that I've seen just call it "sandblast". That's what your gun has.
The actual "Parkerizing" was an experiment limited to 2187 guns (DSC only) in May/June 1942. Details are sketchy, but my suspicion would be that the licensing cost more than the BM but it wasn't found to offer any advantages, so it was discontinued.
In older threads you'll find stories about a proprietary S&W phosphate finish. However, there is no evidence it ever existed.