At the risk of overexposure, there were also some strange claims made in 2009 whose source seems to have vanished but which I recall being based on a different machine ambigram generator, resulting in Cerulean responding with this 7×7 half-turn rotational ambigram grid (where transposing the matrix flips the images, so that there's 28 distinct images presented in 49 cells): https://ceruleanst.livejournal.com/177992.html?nojs=1