'Mystery buffs and historians, for example, can’t find a single straight example of “the Butler did it.”' How about The Cambridge Murders, Glyn Daniel, 1945?
Another very good example of “the Butler did it” is Mary Robert Rinehart’s 1930 novel, “The Door.” Okay, I just kinda blew the ending, but read that one all the same if you’re even remotely interested in mysteries.
But here’s the thing that’s important to note: even by 1930, after which both of these books were written, neither could be the origin of “the Butler did it” cliché because it was considered a cliché even then. In a 1928 essay on detective writing, “Twenty Rules for Writing Detective Stories,” “the Butler did it” was identified as a decades-old cliché never to use. It was already a cliché by the time both of these works were written, and arguably, Rinehart’s book worked because it was playing with the expectation. I haven’t read Glyn Daniel’s book, but my suspicion is that it may play with the expectation also, which was so well ingrained by 1945.