This! I picked up this slate on a recent outing, initially just because I loved its super sharp looking red and black painted border. Then, when the light hit it in a certain way, I realized that the slate on one side is in fact completely covered in what I assume were spelling words (derogation, heteroplasty, pecuniary...) written in pencil so that they more or less disappear unless viewed at a certain angle. Then, I realized one side of the frame was a little loose, and the other side too, and--low and behold!--slid away to reveal hidden notes, which I have to think were essentially little cheat sheets--either answers for a test, or means of remembering certain rules: "Never is island second"; "he a she and is but" etc. It then seems pretty likely that the red and black angled stripes were devised to conceal the clever cuts in the wood frame, and the near-invisible pencil writing on its face to easily evade the surveillance of a teacher or monitor as well. Wow!
My guess on the age of the slate would be c. 1920s-40s or so, and I have to wonder if there are any others out there that feature this adaptation! I might guess that the secret side bars were painted over many times (there is white paint under the pencil text) to allow for new sets of notes to be added! 10 3/4" t x 7 3/4" w x 1/4" thick. There is a hairline crack at the bottom right of the slate visible on the reverse (clean) side, but who cares?!