Based on dsk's reponses to similar questions in recent past he appears to be the forum's metallurgy expert ... but unfortunately must be on vacation or something. However, as far as I know the '40's vintage Colts seem to hold up just fine under one of history's toughest proving grounds ... WWII.
It took me two hours to respond. I do eat and sit on the crapper sometimes, ya know. :hrm:
I would use a 1940 slide only as a restoration project or to sell to somebody interested in same. Prior to the end of WW2 Colt and GI contract slides were only flame-hardened in critical areas, and they would tend to suffer peening damage or stress cracks after thousands of rounds of use. Okay if you're using it for a casual range shooter, not okay if you're planning to use it on an IPSC racegun.