he used X in his letters, 1, 2 or 3 times. I have not seen an O being used, but I am not an expert in the matter. Here is a link to auction house example of such letter: link
looks like he used a lot of due care to get that signature well centered on that cue card... how meticulous he was to do that 500 times over, lol... if you're going to fake someone's signature at least make it look like it was done hastily.