I have Kyle’s book. He was in the innermost in-group of fans and writers and editors in those early days. He knew everyone and appeared everywhere. Almost certainly he would have heard the Campbell story, which as everybody notes Campbell repeated regularly.
A 1976 mention would be the earliest found in print, so that’s a good find. Whether Kyle’s memory was accurate is a question, however. I read thousands of words by Kyle when I was doing the bibliography of Gnome Press, which he co-founded. I seldom cited him because his stories contradicted facts that could be corroborated.
You might think that Alec Nevala-Lee’s gigantic biography of Campbell would settle it. But he calls the story a legend and footnotes Different Engines: How Science Drives Fiction and Fiction Drives Science, by Mark Brake and Neil Hook, which refers to a set of “bright-red” pins around Santa Fe. Los Alamos is also attached to the story in Meltdown: Stories of nuclear disaster and the human cost of going critical, by Joel Levy and in The New Quantum Universe, by Anthony J. G. Hey and Patrick Walters, e.g.
I believe Campbell told the story about the FBI agents’ visit. That could have shut down the magazine and was of gigantic interest to the community. He was a natural story-teller and also a bore who liked to make himself the center of attention. I also believe he embellished the story every time he told it. Can I push that further to say that he changed the location to Oak Ridge at times, when challenged on Los Alamos? Seems possible. I also mentioned Hanford in my OP (original post). But almost all the references I’ve found mention New Mexico. And Kyle’s book was pretty obscure. I’d be amazed if a bunch of writers independently discovered it.
Thanks for the discovery, though. As I implied above, no amount of research ever seems to be enough. A great quote always lies just over the work horizon.
If you need info on other aspects of sf history, I have stacks of nonfiction about the field and I may be able to help. You can message me by clicking on my avatar and clicking the “Message box” in the pop up.
Footnote: I thought I had discovered something when searching fanac.org, a rival site to efanzines.com mentioned above. A 2018 issue of THE MT VOID, the fanzine of the Mt. Holz Science Fiction Society, published endlessly by BNFs (Big Name Fans) Mark and Evelyn Leeper, had this comment on an article: “I heard this story attributed to John W. Campbell years ago. It is probably just an urban legend.” What article? This post on the “‘Straight Dope’s’ bulletin board”. The world is a closed circle. And as we all know, a circle has no end.