Yes, WWII slides were only spot-hardened - at the slide stop notch and the front of the slide, IIRC. On a NOS or freshly parked slide, those two area are typically darker than the balance of the slide. When fired extensively (as in competition), these slides typically crack, along with barrel bushings, slide stops, and the barrels themselves (shearing the 'feet' off).
This is why places like Clark Custom Guns won't do a .460 Rowland conversion on a WWII 1911, or a parts gun with a WWII slide.
"Q: Is it safe to install the 460 Rowland Drop In 1911 Comp Kit into my older spot heat treated 1911 pistol?
A: NO! We do not recommend this kit for installation in any GI or other spot heat treated 1911s!
Please note that we will not guarantee this kit to fit or function in your Auto Ordnance, Llama, Charles Daly or AMT 1911 style pistols nor do we recommend these as platforms for the kit."