Duly noted re: the photo choices. Perhaps they were circumcised free hand, perhaps some other way, perhaps my conjecture is correct. The resolution indeed leaves much to be desired. (FYI,
http://classic-nudists.blogspot.com/?zx=91b48ae846b1a17 is a goldmine of period pics; alas, the site is "the last of the Mohicans" of an online naturist world which the Internet's corporate masters have otherwise cashiered.) However, regardless of my selections, I think my thesis is a missing link. Even with the institutional pressure of the U.S. military (vis. mass enlistment) and the rising trend of in-hospital births mid-century, this doesn't explain prevalence of circumcision. The transition from an intact culture to a circumcised one would have had to be overnight (n.b., "overnight" being c.20 years here). This seems incredible given the Christianity of the times. But if there was a generation or two who were largely pulling back 30-50 years before WWII (just to use the war as a marker), in order to play catch up to WASP pretensions, this makes the popularity of circumcision once the institutions, particularly once the John Rockafeller-trained mainline doctors, endorsed the ritual make more sense.
I'll give another stab at photo evidence; nevertheless, it's the larger idea I find helpful in explaining this bit of American cultural history.