This is great. Most of the info I got on Slack came from Feistel's Lucifer paper, which opens with a short history of iterated (and "product") ciphers. The Germans had a product cipher, ADFGVX, in 1918, but the technology wasn't there to make the design secure.

I looked at Bauer's 'Decrypted Secrets' and when it gets to this it goes - something, ADFGVX, something, Kriegsmarine-modified-Enigma, Shannon and then Suddenly, Horst. It's been a long time since I've read any of the relevant history but it's kind of coming back to me that in the period you picked, cryptography wasn't really a field of significant public research. The Codebreakers was published in '68, just a couple of years before Lucifer. It doesn't even cover the Enigma breaks at all and the publication was considered a national security concern. It seems like not an awful lot about the classified research has come out, even in the decades in which it's surely been publicly replicated and superseded.

I thought they'd be more directly published (rather than inferred) about the technical methods of VENONA but even there, there's very little. Like, they haven't even released the raw decrypts, just translations. And a bunch of papers congratulating themselves on how awesome they were.