In my first post, which was also the FIRST post after the welcome message, I suggested that the aliens use a base of seven. This conjecture is clearly wrong. It was made in haste, when I thought that H might be another digit, possibly the zero. Kevin Stefan suggests a base of six, instead. His arguments are reasonable, and I agree. I also point out that if the base is six, then we could expect that A = 0, B = 1, C = 2, etc. and F = 5. That covers all the digits, and G has been conjectured to be something other than a digit anyway. This is consistent with what I've seen so far, so I am going to use it until I see otherwise.