First I want to thank those who have contributed to the discussion of "President Garfield's Proof". In my initial inquiry, I mentioned that a Garfield biographer has his doubts about Garfield's level of interest and activity in geometry. On the other side, we heard from the archivist at Williams College that there is an 1876 entry in a Garfield journal telling about the proof.
The biographer has consented to my use of his name and response. He is Allan Peskin. His biography of Garfield, published by Kent State University Press, is described at
Here is what Professor Peskin wrote about the 1876 journal entry:
"Of course the Garfield diary for 1876 mentions the proof, but my concern was whether Garfield had devised it as A SCHOOLBOY, as the story sometimes has it. And, in that context, I have found no evidence of any particular interest in geometry in general or that proof in particular that stemmed from his early years. I may have overlooked something, but if so, so did Hendrick Booraem in his otherwise exhaustive study of Garfield's adolescence, THE ROAD TO RESPECTABILITY. In the absence of any such evidence, I decided to omit the tale from my biography. If you find anything that I overlooked, please let me know."