How many rounds did those guns fire? How well and often were they cleaned? There is evidence to suggest that carbon build up in the cone can cause cracking. From the description they were falling apart fast. Like disintegrating. Like... when has this ever happened before with any group of guns? I am always suspicious of the sole example. Was the government aware of the ****** materials being used? They shut down other gun manufacturing during the war for quality issues, why were these passed through?
A nebulous date like "the late 1950s" won't do. I have never seen any proof that model marking indicates the beginning of better, stronger S&Ws. If anyone has such documentation I want to see it to prove my assumption incorrect. Otherwise, I stand by it.
BTW- Here's one of those war production M&Ps made of inferior materials. Has shot countless thousands of rounds including 500 Remington +Ps and 600 of my own +P+s (125@1,150 FPS). Looks like Hell, but mechanically it is perfect after all that.
PS: The barrel was replaced due to a bulge 1" back from muzzle caused by firing with an obstruction. I still have the original barrel and the cone is fine.