It all depends on just how you draw your letters. I see half-turn symmetry in the letters H, I, N, O, S, X and Z. With all of them if you turned them through a half-turn around their center point they would look pretty much the same. Here are the letters right side up and upside down, and that makes me not sure sure. I, N and O are definitely rotationally symmetric, the others look like they might be just a little smaller on the top than the bottom, which would make them almost but not quite symmetric. What do you think?